Designing Electrochemical Energy Storage Microdevices: Li

Designing Electrochemical Energy Storage Microdevices:

Li-Ion Batteries and Flexible Supercapacitors

von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz

genehmigte zur Erlangung des akademischen Grades

doctor rerum naturalium

(Dr. rer. nat.)

vorgelegt von: M.Eng. Wenping Si 司文平

geboren am: August 16, 1986 in Shandong, China

eingereicht am: October 27, 2014

Gutachter: Prof. Dr. Prof. h.c. Oliver G. Schmidt

Prof. Dr. Yongfeng Mei

Tag der Verteidigung: 22 Januar, 2015

Die Dissertation wurde in der Zeit von Oktober 2010 bis September 2014 am IFW Dresden

gefördert durch den International Reseach Training Group (IRTG) und IFW angefertigt.

Bibliographische Beschreibung

Wenping Si

Designing Electrochemical Energy Storage Micro-devices: Li-Ion Batteries and Flexible Supercapacitors

Dissertation (in englischer Sprache)

104 Seiten, 43 Abbildungen, 3 Tabellen, 172 Literaturverweise

Referat

Die Menschheit steht vor der großen Herausforderung der Energieversorgung des 21. Jahrhundert.

Nirgendwo ist diese noch dringlicher geworden als im Bereich der Energiespeicherung und Umwandlung.

Konventionelle Energie kommt hauptsächlich aus fossilen Brennstoffen, die auf der Erde nur begrenzt

vorhanden sind, und hat zu einer starken Belastung der Umwelt geführt. Zusätzlich nimmt der

Energieverbrauch weiter zu, insbesondere durch die rasante Verbreitung von Fahrzeugen und

verschiedener Kundenelektronik wie PCs und Mobiltelefone. Alternative Energiequellen sollten vor einer

Energiekrise entwickelt werden. Die Gewinnung erneuerbarer Energie aus Sonne und Wind sind auf

jeden Fall sehr wichtig, aber diese Energien sind oft nicht gleichmäßig und andauernd vorhanden.

Energiespeichervorrichtungen sind daher von großer Bedeutung, weil sie für eine Stabilisierung der

umgewandelten Energie sorgen. Darüber hinaus ist es eine enttäuschende Tatsache, dass der Akku eines

Smartphones jeglichen Herstellers heute gerade einen Tag lang ausreicht, und die Nutzer einen

zusätzlichen Akku zur Hand haben müssen. Die tragbare Elektronik benötigt dringend

Hochleistungsenergiespeicher mit höherer Energiedichte.

Der erste Teil der vorliegenden Arbeit beinhaltet Lithium-Ionen-Batterien unter Verwendung von

einzelnen aufgerollten Siliziumstrukturen als Anoden, die durch nanotechnologische Methoden hergestellt

werden. Eine Lab-on-Chip-Plattform wird für die Untersuchung der elektrochemischen Kinetik, der

elektrischen Eigenschaften und die von dem Lithium verursachten strukturellen Veränderungen von

einzelnen Siliziumrohrchen als Anoden in einer Lithium-Ionen-Batterie vorgestellt. In dem zweiten Teil

wird ein neues Design und die Herstellung von flexiblen on-Chip, Festkörper Mikrosuperkondensatoren

auf Basis von MnOx/Au-Multischichten vorgestellt, die mit aktueller Mikroelektronik kompatibel sind.

Der Mikrosuperkondensator erzielt eine maximale Energiedichte von 1,75 mW h cm3

und eine maximale

Leistungsdichte von 3,44 W cm3

. Weiterhin wird ein flexibler und faserartig verwebter

Superkondensator mit einem Cu-Draht als Substrat vorgestellt.

Diese Dissertation wurde im Rahmen des Forschungsprojekts GRK 1215 “Rolled-up Nanotechnologie

für on-Chip Energiespeicherung” 2010-2013, finanziell unterstützt von der International Research

Training Group (IRTG), und dem PAKT Projekt “Elektrochemische Energiespeicherung in autonomen

Systemen, no. 49004401” 2013-2014, angefertigt. Das Ziel der Projekte war die Entwicklung von

fortschrittlichen Energiespeichermaterialien für die nächste Generation von Akkus und von flexiblen

Superkondensatoren, um das Problem der Energiespeicherung zu addressieren. Hier bedanke ich mich

sehr, dass IRTG mir die Möglichkeit angebotet hat, die Forschung in Deutschland stattzufinden.

Keywords: Electrochemical energy storage, micro-devices, lithium-ion battery, strain-engineering,

single-rolled up tubes, Si anode, lab-on-chip device, supercapacitor, flexible electronics, solid-state

energy storage

Abstract

Human beings are facing the grand energy challenge in the 21st century. Nowhere has this become more

urgent than in the area of energy storage and conversion. Conventional energy is based on fossil fuels

which are limited on the earth, and has caused extensive environmental pollutions. Additionally, the

consumptions of energy are still increasing, especially with the rapid proliferation of vehicles and various

consumer electronics like PCs and cell phones. We cannot rely on the earth’s limited legacy forever.

Alternative energy resources should be developed before an energy crisis. The developments of

renewable conversion energy from solar and wind are very important but these energies are often not even

and continuous. Therefore, energy storage devices are of significant importance since they are the one

stabilizing the converted energy. In addition, it is a disappointing fact that nowadays a smart phone, no

matter of which brand, runs out of power in one day, and users have to carry an extra mobile power pack.

Portable electronics demands urgently high-performance energy storage devices with higher energy

density.

The first part of this work involves lithium-ion micro-batteries utilizing single silicon rolled-up tubes

as anodes, which are fabricated by the rolled-up nanotechnology approach. A lab-on-chip electrochemical

device platform is presented for probing the electrochemical kinetics, electrical properties and lithium-

driven structural changes of a single silicon rolled-up tube as an anode in lithium ion batteries. The

second part introduces the new design and fabrication of on chip, all solid-state and flexible micro-

supercapacitors based on MnOx/Au multilayers, which are compatible with current microelectronics. The

micro-supercapacitor exhibits a maximum energy density of 1.75 mW h cm-3

and a maximum power

density of 3.44 W cm-3

. Furthermore, a flexible and weavable fiber-like supercapacitor is also

demonstrated using Cu wire as substrate.

This dissertation was written based on the research project supported by the International Research

Training Group (IRTG) GRK 1215 “Rolled-up nanotech for on-chip energy storage” from the year 2010

to 2013 and PAKT project “Electrochemical energy storage in autonomous systems, no. 49004401” from

2013 to 2014. The aim of the projects was to design advanced energy storage materials for next-

generation rechargeable batteries and flexible supercapacitors in order to address the energy issue. Here, I

am deeply indebted to IRTG for giving me an opportunity to carry out the research project in Germany.

September 2014, IFW Dresden, Germany

Wenping Si

Table of contents

1 Background .............................................................................................................................. 1

1.1 Motivation: the energy challenge in the 21st century ................................................... 1

1.2 Aim and structure of this thesis .................................................................................... 3

1.2.1 Single silicon rolled-up tube as anode for micro-batteries ................................ 3

1.2.2 All solid-state and flexible micro-supercapacitors ............................................ 4

2 Introduction to electrochemical energy storage ....................................................................... 5

2.1 Li-ion batteries .............................................................................................................. 6

2.1.1 Introduction to Li-ion batteries .......................................................................... 6

2.1.2 Anode materials for LIBs .................................................................................. 8

2.1.3 Cathode materials for LIBs.............................................................................. 14

2.1.4 Micro/nano-batteries with a single unit of active materials as electrode ........ 16

2.2 Flexible micro-supercapacitors ................................................................................... 18

2.2.1 Introduction to supercapacitors ....................................................................... 18

2.2.2 Fundamentals of electrochemical double layer capacitance and

pseudocapacitance .......................................................................................................... 19

2.2.3 Electrode materials .......................................................................................... 25

2.2.4 Flexible micro-supercapacitors ........................................................................ 29

2.3 Similarities and differences between LIBS and supercapacitors for electrochemical

energy storage ........................................................................................................................ 31

3 Experimental methods ........................................................................................................... 32

3.1 Deposition methods .................................................................................................... 32

3.1.1 Lithography ..................................................................................................... 32

3.1.2 Electron beam evaporation .............................................................................. 33

3.2. Rolled-up nanotechnology .......................................................................................... 34

3.3 Electrochemical measurements .................................................................................. 35

3.3.1 Cyclic voltammetry ......................................................................................... 35

3.3.2 Galvanostatic charge/discharge ....................................................................... 36

3.3.3 Potential step chronoamperometry .................................................................. 36

3.3.4 Electrochemical impedance spectroscopy ....................................................... 37

3.4 Characterization methods ........................................................................................... 38

3.4.1 Scanning electron microscopy ......................................................................... 38

3.4.2 X-ray diffraction .............................................................................................. 39

3.4.3 X-ray photoelectron spectroscopy ................................................................... 39

3.4.4 Atomic force microscopy ................................................................................ 40

3.4.5 Raman spectroscopy ........................................................................................ 40

4. A single rolled-up Si tube micro-battery ............................................................................... 42

4.1 Introduction ................................................................................................................ 42

4.2 Fabrication of a LIB with a single Si rolled-up tube as anode ................................... 44

4.2.1 A single rolled-up Si tube ................................................................................ 44

4.2.2 Assembly of a micro-battery ........................................................................... 45

4.3 Results and discussion ................................................................................................ 46

4.3.1 Characterization of rolled-up Si tube ............................................................... 46

4.3.2 Electrochemical properties of a single rolled-up Si tube ................................. 47

4.3.3 Chemical diffusion and electrical conductivity of a single rolled-up Si tube . 49

4.3.4 Structural observation of Si tube before/after cycling ..................................... 53

4.4 Conclusion .................................................................................................................. 54

5. On chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au multilayers 55

5.1 Introduction ................................................................................................................ 55

5.2 Fabrication of solid-state micro-supercapacitors ........................................................ 57

5.3 Results and discussion ................................................................................................ 58

5.3.1 Schematics of the micro-supercapacitors ........................................................ 58

5.3.2 Oxidation state of Mn ions .............................................................................. 59

5.3.3 Electrochemical performance: CV and EIS ..................................................... 61

5.3.4 Long-term stability, Ragone plot and mechanical flexibility .......................... 66

5.4 Conclusion .................................................................................................................. 68

6. Fiber-shaped supercapacitors based on Cu wire .................................................................... 70

7. Conclusion and Outlook ........................................................................................................ 72

Bibliography ................................................................................................................................. 74

List of Figures and Tables............................................................................................................. 82

Versicherung ................................................................................................................................. 84

Acknowledgements ....................................................................................................................... 85

Publications (Peer-Review) .......................................................................................................... 86

Curriculum Vitae .......................................................................................................................... 88

Acronyms and Abbreviations LIBs Li-ion Batteries

2D Two-Dimensional

EVs Electric Vehicles

HEVs Hybrid Electric Vehicles

EDLC Electrochemical Double Layer Capacitance

EC Ethylene Carbonate

DMC Dimethyl Carbonate

DEC Diethyl Carbonate

PC Propylene Carbonate

CNTs Carbon Nanotubes

SEI Solid Electrolyte Interphase

c-Si/a-Si crystalline Si/amorphous Si

CV Cyclic Voltammetry

TEM Transmission Electron Microscope

ACs Activated Carbon

CNFs Carbon Fibers

OMCs Ordered Mesoporous Carbons

PANI Polyaniline

PPy Polypyrrole

PTh Polythiophene

ILs Ionic Liquids

TEMABF4 Triethylmethylammonium Tetrafluroroborate

PVA Polyvinyl Alcohol

PEO Polyethylene Oxide

PVDF Polyvinylidene Difluoride

PDMS Poly-Demethylsiloxane

PET Polyethylene Terephthalate

PSCA Potential Step Chronoamperometry

EIS Electrochemical Impedance Spectroscopy

SEM Scanning Electron Microscopy

XRD X-Ray Diffraction

XPS X-Ray Photoelectron Spectroscopy

AFM Atomic Force Microscopy

PVD Physical Vapour Deposition

MA 56 Mask Aligner, Version 5.6

QCM Quartz Crystal Microbalance

CPD Critical Point Dryer

ESR Equivalent Series Resistance

SEs Secondary Electrons

BSEs Back-Scattered Electrons

ICDD International Center for Diffraction Data

Symbols and Chemical Formulas D diffusion coefficient

Li/ Li+ lithium/lithium ions

MnOx manganese oxides

Au gold

M molar mass

specific gravity

LixMO2 lithium metal oxides (M=Co, Ni or Mn)

C carbon

Li4Ti5O12 lithium titanium oxides

TiO2 titanium dioxides

Si silicon

Sn tin

e electron

LiPF6 lithium hexafluorophosphate

LiClO4 lithium perchlorate

LiC6 most lithiated graphite

MOx transitional metal oxides (M=Fe, Co, Mo, Cr, Ni, Mo, etc.)

Li2O lithium oxide

Fe2O3, Fe3O4 iron oxides

Co3O4, CoO cobalt oxides

MoO3, MoO2 molybdenum oxides

CuO, Cu2O copper oxides (II, I)

Cr2O3 chromium oxides

NiO nickel oxides

RuO2 ruthenium oxides

Ir2O3 iridium oxide

V2O5 vanadium oxide

PbO2 lead dioxide

NiOOH nickel hydroxide

Ge germanium

Pb lead

Sb antimony

Bi bismuth

Li22Si5, Li15Si4, a-Li3.75Si most lithiated silicon at 415 °C, room temperature, amorphous alloy

LiTiS2 lithium titanium disulfide

LiNi1-yCoyO2 lithium nickel cobalt dioxide

LiNiyMnyCo1-2yO2 lithium nickel manganese cobalt dioxide

LiFePO4 lithium iron phosphate

B boron

Mg magnesium

Al aluminum

Fe iron

Co cobalt

Ni nickel

Zn zinc

SnO2 tin dioxide

ε dielectric constant

A area of electrode surface

C capacitance

π circular constant

d thickness of double layer

Cp, Cn capacitance of positive and negative electrodes

E maximum energy

P maximum power

V operation potential

R total series resistance

H2SO4 sulfuric acid

KOH potassium hydroxide

Na2SO4 sodium sulfate

NH4Cl ammonium chloride

ppm parts per million

RF Faradaic resistance

Cdl double layer capacitance

W Warburg impedance

RE electrolyte resistance

ZF Faradaic impedance

F Faradaic constant

CO2 carbon dioxide

NH3 ammonia

MnO2 manganese dioxide

O2

oxygen ions

OH hydroxyl

K+ potassium cation

Na+ sodium cation

Ag/AgCl silver/silver chloride

GeO2 germanium dioxide

SiOx/SiOy silicon oxide with different oxidation states

Q integrated total charge from CV curve

U potential window of CV scanning

m mass

V volume

m micrometer

cm centimeter

mA milliampere

F Faradaic constant

k0 standard heterophase rate constant

Co, CR concentration of oxidant, reductant

OC bulk concentration of oxidant in electrolyte

I current

Hz unit of frequency

NaCl sodium chloride

keV unit of energy

Ar argon

H2O water

n electron number

uR uncompensated resistance

duCR cell time constant

0r radius of a microelectrode

0

dC capacitance per area

electrolyte conductivity

S cm1

unit of conductivity

ω small sinusoidal pulsation

LiCl lithium chloride

mWh cm3

unit of energy density

W cm3

unit of power density

Chapter 1. Background 1

1 Background

1.1 Motivation: the energy challenge in the 21st century

Energy ranges among the most important topics in the 21st century. Optimistic predictions

forecast that a peak production of oil, coal, and natural gas will happen in 2020s or 2030s and

alternative energy resources should be developed before a crisis. It is human being’s challenge in

this century to be independent from the earth’s legacy and to build renewable clean energy

resources to enable the sustainable development of our economy and society, making sure that

our descendent can live in peace. Solar and wind energy as representative renewable energy have

attracted unprecedented interest since they provide electricity without giving rise to any carbon

dioxide emission. However, the production of solar and wind energies is not even and continuous

due to their intermittent nature, i.e., significant dependence on natural conditions (day time, night

time, wind, etc.). In order to utilize electricity from such kinds of energy, some corresponding

energy storage approaches, such as thermal, mechanical, electromagnetic, hydrogen, and

electrochemical energy storage, are necessary to store the energy and to stabilize the electricity

grid connected. As an intermediate step towards the utilization of renewable clean energies,

energy storage systems are crucial supporting facilities. On the other hand, the increasing

popularization of consumer electronics and electric vehicles also greatly promotes the

development of energy storage devices. Among all the energy storage approaches,

electrochemical energy storage devices play an important role due to their high energy/power

density, versatility, and flexibility. However, energy storage cannot keep pace with the progress

in the microelectronic industry (Moore’s law predicts a doubling of memory capacity every two

years) and it has been the bottleneck for the further reduction of the size of microelectronic

devices.

Electrochemical energy storage/release is realized by electron and ion charge/discharge. Two

typical electrochemical energy storage devices are rechargeable batteries[1] and electrochemical

capacitors (also named supercapacitors).[2] The advantage of electrochemical energy is

described in the Ragone plot (Figure 1. 1), where typical energy storage and conversion devices

2 Chapter 1. Background

are presented in terms of their specific energy and specific power densities. Batteries and

electrochemical capacitors bridge the gap between fuel cells and conventional capacitors.

Furthermore, batteries usually exhibit higher energy density while supercapacitors have higher

power density. Thus, the main focus in this thesis includes Li-ion batteries (LIBs, specifically on

the anode materials) and supercapacitors.

Figure 1. 1. Sketch of Ragone plot shows specific power against specific energy for various electrical energy

storage devices. The specific power indicates how fast it can be charged/discharged, and the specific energy

indicates how much energy it supplies on a single charge. Cited from Ref. [3].

Strain engineering offers an advanced strategy to deterministically rearrange 2D (two-

dimensional) nanofilms into 3D micro-/nanostructures including tubes, helices, rings, wrinkles

and other advanced micro-architectures. In our group, we have reported rolled-up microtubes as

electrodes in Li-ion batteries and supercapacitors, all of which have shown high performance

(e.g., capacity and life time).[4-9] The next section gives an insight into the aim of the first part

in this work utilizing the rolled-up nanotechnology approach to fabricate single silicon rolled-up

tubes as anodes for micro-batteries.

The recent rapid advance and eagerness of miniaturized, portable consumer electronics

stimulate the development of micro-scale power sources with high power density, towards the

Chapter 1. Background 3

trend of being small, thin, lightweight, flexible, and even wearable, to meet the growing demands

of modern society.[10-13] Introducing small solid state energy storage devices has stimulated

significant research interest, which enables circuit designers to integrate energy storage devices

directly with other functional components or even on clothing. However, it is still a challenge to

realize flexible energy storage devices with high performance because they are highly dependent

on the electrical and mechanical properties of electrode materials. The next section explains the

aim of the second part in this thesis to fabricate all solid-state, flexible supercapacitors, and the

structure of this thesis.

1.2 Aim and structure of this thesis

1.2.1 Single silicon rolled-up tube as anode for micro-batteries

It is well known that once a two-dimensional nanofilm is rolled up into a microtube, both the

upper and lower surfaces will be exposed to electrolyte, and the interior space of microtubes

could work as advanced ionic transport channels. A particular method based on rolled-up

nanotechnology has been recently developed in the IIN institute. With this method it is possible

to fabricate tubular structures with various sizes which enable the integration of lab-on-a-chip

electrochemical devices.

A comprehensive understanding of the correlation between the electrodes’ tubular structure,

electrical/ionic conductivity and the electrochemical kinetics as well as the performance is

needed to be explored. Ionic conduction through the electrodes and electrolyte is essential to

complete the electrochemical reaction.[14] An important kinetic characteristic is the chemical

diffusion coefficient (D) of the inserted species (Li), which often determines the total reaction

rate in the kinetic diffusion process. Thus, the determination of D values of Li ion in the solid

phase is a most important issue from a practical as well as fundamental point of view. In addition,

the ease of electron-transfer between anode and cathode can dictate the magnitude of the cell’s

driving force, which is related with electrodes, current collectors and electrical leads. Therefore,

this work will focus on the understanding of the ionic and electrical conduction of the Si single

tubes.

4 Chapter 1. Background

1.2.2 All solid-state and flexible micro-supercapacitors

In comparison with Li-ion batteries, supercapacitors have many unique advantages such as high

power density, long lifetime, easy fabrication, low cost and good safety.[3, 15, 16] With the

rapid development of the multifunctional portable consumer electronics and energy harvesting

devices such as solar cells, energy storage components have become a bottleneck for the

reduction of the size of microelectronic devices. There is high demand to produce miniaturized,

flexible and even weavable supercapacitors. Therefore in this work, a new concept is introduced

to fabricate on chip, all solid-state and flexible micro-supercapacitors based on MnOx/Au

multilayers, which are compatible with current microelectronics. The micro-supercapacitor

exhibits a maximum energy density of 1.75 mW h cm-3

and a maximum power density of 3.44 W

cm-3

. Furthermore, a flexible and weavable fiber-like supercapacitor is demonstrated using Cu

wire as substrate.

Chapter 2. Introduction to electrochemical energy storage 5

2 Introduction to electrochemical energy

storage

Electrochemical energy storage devices, of which the two dominant kinds are rechargeable

batteries and supercapacitors, store and release their energy by electron and ion charge/discharge

in electrochemical processes. An electrochemical energy storage device is usually composed of a

positive (cathode) and a negative electrode (anode), separated by a separator and electrolyte

solution. When connected to an external circuit, during discharging, the electrochemical

reactions occur in series at both the cathode and the anode, generating electrons and enabling the

current to be captured by the user; during charging, an external voltage is applied across the

electrodes, driving the movements of electrons and reactions at the electrodes and realizing the

energy storage.

According to the mechanisms and components of their electrodes, rechargeable batteries can

be further separated into the following categories: lead-acid, zinc-air, nickel-cadmium, nickel-

hydrogen, sodium-sulfur, sodium-nickel-chloride, and LIBs.[1] Rechargable LIBs offer energy

densities 2-3 times and power densities 5-6 times higher than Ni-MH, Ni-Cd, and Pb acid

batteries. Due to the advantages of high voltage, low self-discharge, long cycling life, low

toxicity, and high reliability, LIBs have been one of the most important energy storage system

for a wide variety of applications in the communications, transportation and smart grid, such as

portable electronic devices including cell phones, laptops, and digital cameras, as well as electric

vehicles (EVs) and hybrid electric vehicles (HEVs).[17-19] This explains why they receive the

huge attention at both the fundamental and practical levels.

Supercapacitors, featured with high power capabilities, have also attracted increasing interest

especially for applications in EVs and HEVs, as well as portable electronics. Two principles are

responsible for supercapacitors: electrochemical double layer capacitance (EDLC) and pseudo-

capacitance modes. The former, similar to an electrolytic capacitor, is working by separating

charges at the interface between a solid electrode and an electrolyte; and the latter is a fast

Faradaic process involving electrochemical redox reactions.

The electrode materials play key roles in determining the performance of LIBs and

supercapacitors. Understanding charge storage mechanism and the development of advanced

6 Chapter 2. Introduction to electrochemical energy storage

nanostructured materials, synthesis techniques have promoted notable improvements in the

performance of LIBs and supercapacitors. Herein, detailed overviews of the advanced materials

for LIBs and supercapacitors are presented in 2.1 and 2.2, respectively.

2.1 Li-ion batteries

2.1.1 Introduction to Li-ion batteries

Li is the most electropositive (3.04 V vs. standard hydrogen electrode) and the lightest (molar

mass M=6.94 g mol1

, and specific gravity =0.53 g cm3

) metal, facilitating the development of

batteries with Li metal as anode. In 1970s, the assembly of primary (non-rechargeable) Li cells

firstly demonstrated the advantage of using Li metal.[17, 20]. They emerged soon as power

sources for watches, calculators or for implantable medical devices due to their high energy

capacity and variable discharge rate. Meantime, numerous inorganic compounds were

demonstrated to react with alkali metals reversibly, which were then taken as positive electrode

in primary Li cells. Bell Labs discovered that oxides as intercalation materials were giving

higher capacities and voltages. Goodenough proposed the families of LixMO2 compounds (M is

Co, Ni or Mn) that are still used as cathodes in today’s batteries.[21, 22] But shortcomings soon

occurred since the dendritic Li grew during repeated discharge/charge cycles, which led to

explosion hazards. An effective approach is replacing Li metal with an insertion material. At the

end of the 1980s and early 1990s, Li-ion battery has developed. The change in the presence of Li

from its metallic state to ionic state solved the dendrite problem, making LIBs inherently safer

than Li-metal cells. To compensate for the increase in the potential of the negative electrode,

high-potential insertion compounds are necessary for the positive electrode, and thus the

emphasis shifted from the layered-type transition-metal disulphides to layered or three-

dimensional-type transition metal oxides.[21]

Fundamentally, LIBs’ capacity and lifetime are determined by the intrinsic properties of the

positive and negative electrode materials. Currently researchers aim to find the best-performing

combination of electrode-electrolyte-electrode through selecting of cathodes and anodes as well

as the appropriate electrolytes, meanwhile minimizing detrimental reactions associated with the

electrode-electrolyte interface.


Figure 2. 1. Schematic operating mechanisms of a typical rechargeable LIB containing a silicon anode and lithium

metal oxide cathode during a) charging and b) discharging. Cited from Ref.[23]

A typical LIB, as shown in Figure 2. 1, consists of a cathode (positive) and an anode

(negative), together with an electrolyte-filled separator that allows the transfer of lithium ions but

prevents electrodes from direct contact. The positive electrode (cathode) materials are typically

layer-structured metal oxides containing Li (such as LiCoO2) or tunnel-structured materials (such

as LiMn2O4). The negative electrode (anode) materials can be separated into several categories:

insertion-type materials (such as C, Li4Ti5O12, TiO2, etc.), conversion-type materials (such as

iron oxides, nickel oxides, cobalt oxides, etc.), and alloying-type materials (such as Si, Sn, etc).[1]

Taking LiCoO2 as a cathode and Si as an anode in a typical LIB, the complete chemical reactions

are described as follows:

Anode:

SiLixexLiSix

(2. 1)

Cathode:

xexLiCoOLiLiCoO

x 212 (2. 2)

As for the electrolyte, it should have good ionic conductivity and safety. The most commonly

used electrolytes for LIBs are inorganic lithium salts, such as LiPF6, LiClO4, etc., dissolved in a


mixture of two or more organic solvents (ethylene carbonate (EC), dimethyl carbonate (DMC),

diethyl carbonate (DEC), propylene carbonate (PC), etc.). The separator has two functions: to

prevent short circuiting between the anode and cathode, and to provide abundant channels to

transport Li ions during charging/discharging as well. A typical separator for LIBs is the glass

membrane. By the exchange of Li ions between the anode and cathode, the chemical energy and

electrical energy can be converted reversibly. Thus LIB is also named as rocking-chair battery as

Li ions rock back and forth between the anode and cathode. When the battery is charging, Li

deintercalates from the cathode and intercalates into the anode. Vice versa, Li deintercalates

from the anode and intercalates into the cathode during discharging.

2.1.2 Anode materials for LIBs

Insertion-type materials (C, Li4Ti5O12, TiO2):

The insertion-type materials have advantages such as low cost and nontoxicity. The main

problem is that the number of electrons involved in the insertion reaction is generally less than

one per Li because Li can only be accommodated into interstitial sites of the anode, thus the

capacity is low. Here carbon materials are introduced.

Figure 2. 2. A schematic drawing of graphene. Reproduced from Ref. [24].

Graphite is the most commonly used commercial anode materials for LIBs because of its low

and flat working potential, long cycle life, and low cost. The maximum lithiated compound of

graphite is LiC6, resulting in a limited theoretical charge capacity of 372 mAh g1

. Furthermore,

the Li ion transport rates of graphite anodes are always less than 106

cm2 s1

, leading to a low

power density.[18] Nanostructured carbonaceous anode materials have shown potentials to

increase the energy and power densities of LIBs, including 1D, 2D, and porous carbon-based


anodes, since they possess larger surface area and more open spaces for Li storage.[18] 1D

nanostructured carbon includes carbon nanotubes (CNTs), nanowires, and nanofibers. For

instance, the reversible capacity of anodes made from CNTs can exceed 681 mAh g1

.[25] A

typical 2D nanostructured carbon is graphene (Figure 2. 2), a novel monolayer of carbon lattice,

which has an ultrahigh surface area, intriguing electronic and thermal conductivities, structural

flexibility, unique porous structure, and a broad electrochemical window.[26] It was found that

graphene exhibits a relatively high reversible capacity of 672 mA h g1

and fine cycle

performance.[27] 3D porous carbon materials are also considered as promising anode materials

for LIBs since they have high surface areas and open pore structures. Mainly there are three

categories of them: microporous (pore size < 2 nm), mesoporous (2 nm < pore size < 50 nm) and

macroporous (pore size > 50 nm). The mesoporous carbon exhibits the largest reversible capacity

of 1100 mA h g1

.[18, 28] The electrochemical performances of nanocarbons (1D, 2D and

porous carbons) as anode in LIBs are dependent on the structures and morphologies. The 1D

nanostructured carbons usually have high Coulombic efficiency (the ratio of the charge capacity

to the discharge capacity for each cycle) and good cycling stability. However, they often show

bad rate capability due to the formation of large SEI (solid electrolyte interphase) film and less-

compact structure. 2D structured graphene typically shows good rate capability, but has low

initial Coulombic efficiency and large irreversible capacity. Porous carbons have low volumetric

capacity and large irreversible capacity. Hybridizing nanocarbon materials with other high-

capacity components (alloys and/or metal oxides), could lead to improved electrochemical

performances.

Conversion-type materials

Conversion-type materials have been intensively investigated as potential anode materials for

rechargeable LIBs because these materials can deliver high reversible capacities between 500

and 1000 mAh g1

.[18] Most transitional metal oxides (MOx, M=Fe, Co, Ni, Cu, Mo, Cr, Ru, etc.)

follow the conversion mechanism, which involves the formation and decomposition of lithium

oxide (Li2O), accompanying with the reduction and oxidation of metal nanoparticles, displayed

as follows:[18]

OyLixMyeyLiOMyX 2

22 (2. 3)


After the first lithiation, the metal oxides are converted to a metallic state along with the

formation of Li2O, and reversibly returned back to its initial state after delithiation. However,

they often show low Coulombic efficiency at the first cycle, unstable SEI film formation, large

potential hysteresis, and poor capacity retention. In order to address the issue, nanostructured

porous transitional metal oxides and their composites have been investigated. Table 2. 1

summarizes the most widely studied conversion reaction-based transitional metal oxide anodes.

Table 2. 1. Summarization of conversion reaction-based nanostructured transitional metal oxide anodes. Modified

based on Ref. [18].

Metal oxides Theoretical capacities

(mAh g1

)

Representative nanostructures Common problems and

possible solutions

Iron oxides Fe2O3

Fe3O4

1007[29]

926[30]

Nanostructures and carbon-

based nanocomposites

Common problems:

Low coulombic

efficiency at the first

cycle, unstable SEI

film formation, large

potential hysteresis,

and poor capacity

retention.

Possible solutions:

1. Metal oxide/carbon

composites using

carbon as buffer and

electrode-active

materials.

2. Nanostructured

metal oxides to

provide high surface

area and quantum

confinement effects.

Cobalt oxides Co3O4

CoO

890[31]

715[32]

Nanostructured Co3O4 with or

without carbon

CoO composites

Manganese oxides MnOx 700-1000[33, 34] Nanostructured MnOx/carbon

Molybdenum

oxides

MoO3 1111[35] Doped MoO3

MoO2 830[35] MoO2 nanomaterials

Copper oxides CuO 674[36] Nanostructured CuO

Cu2O 375[37] Cu2O/carbon composites

Chromium oxides Cr2O3 1058[38] Nanostructures, hetero-atom

doping, and carbon-based

nanocomposites

Nickel oxides NiO 718[39] NiO/carbon, porous NiO

Ruthenium oxides RuO2 1130[40] SnO2/RuO2

Alloy-type materials

Other elements such as Si, Ge, Sn, Pb, Sb and Bi and their alloys or oxides, have been examined

as anode for LIBs as they deliver much higher capacity than both insertion- and conversion-type

anodes. Amongst them, silicon based anodes for LIBs have been the focus of intensive research

interests, primarily because silicon exhibits the highest theoretical specific capacity (4200 mAh

g1

for Li22Si5), more than ten times of that of carbon [41] and is among the most promising

candidates that can replace graphite as anodes in rechargeable Li-ion batteries. Crystalline Si is


in the mFd3 space group, the face-centered cubic bravais lattice. It is worth noting that the

Li22Si5 compound is only obtained at a high temperature of 415 °C,[41] while Li15Si4 (or

amorphous phase a-Li3.75Si) is the highest lithiated phase achievable at room temperature for the

lithiation of silicon, corresponding to a capacity of 3579 mAh g1

.[42-46]

Figure 2. 3. Schematic illustration of Si nanowire anode grown directly onto the current collector, reproduced from

Ref. [47].

However, a huge volume change of Si (280%) [44, 48] also accompanies the lithiation-

delithiation process, which leads to the loss of contact or pulverization of the electrodes upon

repeated alloying-dealloying, and eventually limits the use of Si in commercial batteries.

Amorphous Si exhibits much improved cycling performance over crystalline Si due to less

dramatic volume changes.[43] Nanostructured Si as well as their composites with carbon (Si/C)

materials also have shown promising resistance against fracture and exhibited improved cycling

performance, including Si nanoparticles,[49] Si nanowires/nanorods,[47, 50] Si nanotubes[51]

and Si nanospheres.[52] However, these nanomaterials also show some disadvantages, such as

low thermodynamic stability and surface side-reactions, which are related to the small size and

high specific surface area (i.e., excess surface free energy).[53] Therefore kinetically stabilized

nanomaterials should be considered, which can be realized by using nano/micro hierarchical

structures and proper surface coating.

To effectively solve the issue of poor cycle life, it is also of great importance to fully

understand the detailed lithiation/delithiation processes of Si. Researchers have tried various

strategies to determine the complicated electrochemical redox reactions during alloy/dealloy

process of Li and Si. Hatchard et al. used in-situ XRD (X-ray diffraction) to track the reaction of

lithium with amorphous Si.[43] Li et al. have studied the sloping plateaus behaviors (see Figure


2. 4) using Sn-doped a-Si powders.[54] Real-time NMR (nuclear magnetic resonance) technique

was utilized by Key et al[55] to investigate the structural changes in Si during lithiation and

delithiation. Following Huang’s pioneering work of real time observation of the lithiation

process of SnO2 by in-situ TEM (transitional electron microscopy),[56] several powerful

experiments have also been carried out on Si by in-situ observation.[57, 58] For example, a two-

phase electrochemical lithiation process was detected in amorphous Si.[58] However,

controversies on the lithiation/delithiation mechanisms of Si still exist. According to diverse

literatures results,[43-45, 47, 54, 55, 57-71] Zamfir et al. have managed to draw a conclusion as

shown in the following table.[46] Here crystalline Si is simplified as c-Si, and amorphous Si as

a-Si. The first lithiation of c-Si involves a two-phase reaction (c-Si and a-Li3.5Si) and then an

amorphous Li3.75Si or a crystalline Li15Si4 is produced as the fully lithiated phase. The first

delithiation of c-Si includes three steps, firstly a two-phase reaction (the fully lithiated Si and a-

Li2Si), secondly a less lithiated amorphous phase (a-LixSi, 0<x<2), and finally the amorphous Si.

Until now, the originally amorphous Si has been amorphized and the afterwards

lithiation/delithiation processes are identical to those of amorphous Si. There are three steps in

the first lithiation of amorphous Si (also the second lithiation of c-Si). Firstly a two-phase

reaction (a-Si and a-Li2.2Si) occurs, secondly another more complicated amorphous region occurs

(still not clear if it is a two-phase region), and finally Si is fully lithiated to either a-Li3.75Si or c-

Li15Si4. The delithiation process of a-Si is identical to that of c-Si.


Table 2. 2. Summary of the lithiation/delithiation mechanisms for crystalline Si (c-Si) and amorphous Si (a-Si).

Reproduced from Ref. [46].

First full lithiation of c-Si

First delithiation of c-Si

Second full lithiation of c-Si (first lithiation

of a-Si)

Second delithiation of c-Si (first delithiation

of a-Si)

415

753

53Sic-Li

Sia-LiSi a-LiSi cSi c

.

.

100 mV, 2 phases

a-SiSia-LiSia-LiSic-Li

Sia-Li

Sic-Li

Sia-Li

tt

..

202

415

753

415

753

430 mV, 2 phases

415

753

5252SiLic

SiLiaSiLiaSiLiaSiLiaSiaSia

.

w(?)..

1st sloping plateau 2

nd sloping plateau

2 phases 2 phases?

a-SiSia-LiSia-LiSic-Li

Sia-Li

Sic-Li

Sia-Li

tt

..

202

415

753

415

753

430 mV, 2 phases

The typical galvanostatic profiles and cyclic voltammetry (CV) curves of crystalline Si

nanowires are shown in Figure 2. 4. The first lithiation shows one gently sloping plateau

corresponding to the equilibrium between c-Si and a-Li3.5Si. At the end of the first lithiation (0

V), c-Si has been totally transformed into a-Li3.75Si, which is also possibly crystallized into c-

Li15Si4 phase, depending on conductivity and other parameters. The first delithiation starts with a

steep rise in potential, followed by a plateau at ~ 430 mV, corresponding to the equilibrium

between c-Li15Si4 (or a-Li3.75Si) and a-Li2Si. At the end of the delithiation step, Li is totally

extracted from the alloy and Si is left amorphous. The second and third lithiation profiles exhibit

different behaviors with two sloping plateaus, which is typical lithiation feature of amorphous Si.

The delithiation processes of the first three curves are similar. CV shows one sharp peak for the

first lithiation, corresponding to the gently sloping plateau. The second lithiation produces two

peaks, corresponding to the two sloping plateaus in galvanostatic curves. The first and second

delithiation curves are similar, exhibiting two peaks at approximately the same potential.


Figure 2. 4. Galvanostatic and CV curves recorded on crystalline Si nanowires in a half-cell geometry with Li

counter electrode. (a) The first galvanostatic lithiation shows a gently sloping plateau, starting at around 100 mV. (b)

The first three galvanostatic lithiation curves show the difference in lithiation behaviors between the first cycle (c-Si)

and the following ones (a-Si). The inset shows the first two cycles of CV. Reproduced from Ref. [46].

2.1.3 Cathode materials for LIBs

Since anodes of LIBs have no Li, cathodes then must act as a source of Li, thus requiring use of

air-stable Li-based intercalation compounds to facilitate the cell assembly.[17] Cathode materials

incorporate two main classes of materials.[72] The first group contains layered compounds,

which are made of two types of alternative layers, one of close-packed anions and the other of

the transition metals. Li ions can insert into essentially empty remaining layers, as shown in

Figure 2. 5a. Typical examples of this group are LiTiS2, LiCoO2, LiNi1-yCoyO2, and

LiNiyMnyCo1-2yO2. The second group possesses more open structures, including many vanadium

oxides, the tunnel compounds of manganese dioxides, and transition-metal phosphates, such as

olivine LiFePO4 (see Figure 2. 5b). The first group have higher specific volumetric energy

density due to their more compact lattices, but LiFePO4 in the second group have advantages in

lower cost and higher rate capability.[72] The most widely used cathode materials include

LiCoO2, LiMn2O4, LiFePO4.[1]


Figure 2. 5. Schematic structures for two groups of cathodes. (a) Layered structure of the first group, such as LiTiS2,

LiCoO2. Reproduced from Ref. [72]. (b) Olivine structure of LiFePO4 in projection along [001]. Reproduced from

Ref. [17].

LiCoO2 can be facilely fabricated with desirable electrochemical properties (good structural

stability and a moderately high capacity of 140 mAh g1

with a cut off voltage of 4.2 V).[17] But

the major drawbacks of this material are high cost and toxicity.[73, 74] Due to the low cost of

manganese oxides, the use of them in LIBs has been stimulated. The discharge of LixMn2O4

occurs at around 4 V and 3V versus Li/Li, resulting in Li2Mn2O4.[34] The challenge facing the

pristine LiMn2O4 is the severe capacity fading upon cycling. This problem can be tackled by the

substitution of different cations (Li, B, Mg, Al, Fe, Co, Ni, or Zn) or by the introduction of

nanodomain structures.[75, 76] Benefiting from the advantages of potentially low cost, rich

resources, and environmental friendliness, LiFePO4 has also attracted much attention. The

discharge potential of LiFePO4 is about 3.4 V versus Li/Li. Moreover, nearly 90% of its

theoretical capacity can be used and no obvious capacity fading is observed for this material

even after several hundred cycles. Its capacity approaches 170 mAh g1

, which is higher than that

of LiCoO2 and comparable to stabilized LiNiO2. However, this material has a very low electrical

conductivity at room temperature. In order to achieve its theoretical capacity, the current density

has to be limited at a very low value.[77] By carbon coating, metal-rich phosphide nano-

networking, super-valence ion doping, and aliovalent substitution, the conductivity of LiFePO4

can be improved considerably.[78, 79]


2.1.4 Micro/nano-batteries with a single unit of active materials as

electrode

Bulk batteries are often composite electrodes made by mixing active materials with conductive

carbon additives and polymer binders, which introduce uncertainties into electrochemical kinetic

study. In order to eliminate the interference, the investigation of a single unit of active material

with well-defined geometry should be an attractive approach. In the field of nanoscience, single

nanostructured electronic devices have been extremely useful for investigating electronic

properties of nanomaterials.

Figure 2. 6. Schematic of measuring a single Si nanowire as anode for LIB. Reproduced from Ref. [80].

Since the research objects are very small, realizing the electrical contact to them is challenging.

A schematic of measuring one single Si nanowire is shown in Figure 2. 6. The electrical contact

was realized by Ti metal strips deposited either using e-beam evaporator or sputtering techniques.

With this platform, McDowell et al. succeeded in measuring both the electrochemical properties

and the electrical transport of the single Si nanowire under various lithiation states.[80] Mai and

his co-workers [81] also used the similar platform to study the evolution of single silicon

nanowire, with the ability to in situ record the electrical transport of the single nanowire.


Figure 2. 7. Schematic of measuing a single particle anode in a LIB. Reproduced from Ref. [82].

The electrical contact to a single particle was achieved by a microelectrode connected with a

micromanipulator under optical microscopy (Figure 2. 7). With the similar setup, Uchida’s group

[83, 84] investigated the kinetics of Li+ extraction/insertion at a single particle (graphitized

mesocarbon microbeads and LiMn2O4). This technique enabled studying the current/potential

behavior of the particle itself.

Figure 2. 8. (a) Schematic of the experimental setup of observing the lithiation behaviors of SnO2 nanowires under

TEM. IL means ionic liquid as electrolyte. (b) TEM micrograph of the nanowire containing a reaction front

(dislocation cloud) separating the reacted (amorphous) and nonreacted (single-crystal SnO2) sections. Reproduced

from Ref. [56].

Another way to contact single nanowires was realized by connecting the nanowires with Au

rod by a conductive epoxy as shown in Figure 2. 8a. Huang and co-workers [56] have utilized

such a nanoscale electrochemical device inside a TEM to observe the in-situ lithiation of the tin

dioxide (SnO2) nanowire during electrochemical charging and found that a reaction front

containing a high density of dislocations worked as a structure precursor to electrochemically

driven solid-state amorphization. And Wang et al., [58] further reported a two-phase


electrochemical lithiation at amorphous Si with the similar setup.

In this thesis, a robust Lab-on-chip electrochemical device platform is presented for probing

the electrochemical kinetic, electronic properties and structural changes study of a single silicon

rolled-up microtube as anode in LIBs. This will be discussed in details in Chapter 4.

2.2 Flexible micro-supercapacitors

2.2.1 Introduction to supercapacitors

Supercapacitors, the term of which was coined by B. E. Conway,[2] find versatile applications

due to their high power ability since they store energy using either EDLC or rapid redox reaction

(pseudo-capacitance). Supercapacitors can be fully charged and discharged in seconds and

deliver an energy density of about 5 Wh kg1

(lower than batteries), but a much higher power

density (10 kW kg1

),[2] as shown in Figure 1. 1 in Chapter 1. As macro-supercapacitors, they

are used in hybrid electric vehicles to increase the efficiency. They can complement or replace

batteries in electrical energy storage applications.[16] Since today’s hybrid vehicles turn off the

engine completely when the car comes to a stop, supercapacitors can offer efficient power for a

rapid restart. The use of supercapacitors in emergency doors (16 per plane) on an Airbus A380

also confirms their performance, safety and reliability.[16] As micro-supercapacitors, they can be

integrated as energy storage devices with portable consumer electronics.

Figure 2. 9. Schematic illustration of three types of supercapacitors, based on the electrode materials and their

working principles.


According to the two types of working principles (electrochemical double layer and pseudo-

capacitance), supercapacitors can be divided into three types, as summarized in Figure 2. 9. (a)

Electrochemical double layer capacitors, similar to physical capacitors, work by the separation of

charges at the interface between a solid electrode and an electrolyte, which are non-faradaic

processes. Carbon materials are the most widely used electrodes for this type of supercapacitors,

including activated carbon (ACs), carbon fibers (CNFs), carbon aerogel and carbon nanotubes

(CNTs), ordered mesoporous carbons (OMCs) and newly discovered graphene, carbide-derived

carbon, and carbon onions; (b) Pseudo-capacitors often involve rapid faradaic redox reaction

either on surface or in bulk of the electrodes. Two main kinds of electrode materials following

this mechanism are transition metal oxides such as RuO2, MnO2, NiO, Co3O4, and conducting

polymers including polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) and its

derivatives; (c) Hybrid capacitors (supercabattery) combine one battery-like electrode and one

supercapacitor electrode, resulting in improved energy density as well as power density.[16] The

following section will give detailed descriptions about the fundamentals and various electrode

materials of supercapacitors.

2.2.2 Fundamentals of electrochemical double layer capacitance and

pseudocapacitance

Double layer capacitance utilizes delocalized conduction-band electrons of electrodes, while

faradaic processes involve electrons transferred to or from valence-electron states (orbitals) of

electrodes. In certain cases, the faradaically reactive material itself is metallically conducting

(e.g., PbO2, some sulfides, RuO2), or is a well-conducting semiconductor and a proton conductor,

e.g., Ni O OH.[2]

A supercapacitor, with two electrode/electrolyte interfaces, can be considered as two

capacitors connected in series (see Figure 2. 10a). The capacitance of each interface can be

expressed as follows:

d

AC

4 (2. 4)

where A is the area of the electrode surface, which should be the active surface of the electrode

porous layer, ε is the medium (electrolyte) dielectric constant, which will be equal to 1 for a

vacuum and larger than 1 for all other materials, including gases; and d is the effective thickness


of the electrical double layer.[15] The capacitance can be obtained experimentally by CV or

galvanostatic charge/discharge measurements, as described in Chapter 3.3.

When the specifications of a supercapacitor are given, it should be clarified whether the

values correspond to a single electrode measurement or a complete capacitor. If the capacitance

of the positive and negative electrodes can be expressed as Cp and Cn, the overall capacitance can

be expressed as the following equation:[15]

npCCC

111 (2. 5)

If these two electrodes are the same, i.e., symmetric supercapacitor, the overall capacitance

would be half of either one’s capacitance. If two electrodes are different, the electrode with

smaller capacitance determines the overall capacitance (the same for LIBs).

The maximum energy (E) and power (P) stored in such a capacitor is given by

221 CV/E (2. 6)

RVP 4/2 (2. 7)

Here V is the operation potential of the capacitor and R represents the total series resistance.[3]

All these values can be obtained by CV, galvanostatic charge/discharge and electrochemical

impedance spectroscopy measurements, which will be described in Chapter 3.3.

The principle of double layer capacitance

Figure 2. 10a shows a charged EDLC at open circuit, with two potential drops across the

electrode/electrolyte interfaces. For aqueous electrolyte-based EDLC, the highest operating

potential is 1.2 V, and the potential could reach up to 4 V for organic electrolyte-based EDLC.

The double layer is generally referred to a simplified model—the compact Helmholtz layer, with

a thickness of 2-10 Å. Actually the full double layer should include another diffusion layer with

a certain distance to the electrode/electrolyte interface, the so-called Stern model, as shown in

Figure 2. 10b. Because ions on the solution side of the double layer would not remain static in a

compact array, but would be subject to the effects of thermal fluctuation according to Boltzmann

principles.[2] The Helmholtz layer and diffusion layer are conjugate components (in series) of


the overall double layer. The smaller one of the two capacitances determines the overall double

layer capacitance.[2]

Figure 2. 10. Schematic diagrams (a) a single-cell double layer capacitor and the illustration of the potential drop at

the electrode/electrolyte interface. Reproduced from Ref. [3]. (b) Stern model of the double layer for finite ion size

with thermal distribution, combining Helmholtz and diffusion layers. Reproduced from Ref. [2].

The principle of pseudocapacitance

Pseudocapacitance arises at electrode surfaces, where a charge storage mechanism different from

the EDLC applies. Fast and reversible faradaic (redox) reactions take place on the electrode

materials and involve the passage of charges across the double layer, similar to the charging and

discharging processes that occur in batteries.

Figure 2. 11. The working principle of pseudocapacitors. (a) Redox reaction on surface; (b) redox reaction in bulk.

Reproduced from Ref. [1].

Three types of faradaic processes can be distinguished: reversible adsorption (e.g., adsorption

of hydrogen on the surface of platinum or gold), redox reactions of transition metal oxides and

reversible electrochemical doping-dedoping in conducting polymer based electrodes.[2] In other


words, two processes occurring on both surface and bulk are responsible for pseudocapacitance.

Thus pseudocapacitors exhibit a capacitance 10-100 times higher than EDLC.[2] Schematic

illustrations describing the two processes are shown in Figure 2. 11.

For the hybrid supercapacitor, both EDLC and faradaic capacitance mechanisms occur

simultaneously, but one of them determines the overall performance. In both mechanisms, large

surface area, appropriate pore-size distribution, and high conductivity are essential properties of

the electrode materials to achieve large capacitance.

Electrolyte

The electrolyte in a supercapacitor requires the following properties: wide voltage window, high

electrochemical stability, high ionic concentration and low solvated ionic radius, low resistivity,

low viscosity, low volatility, low toxicity, low cost as well as availability at high purity.[15] Four

types of electrolytes can be used in supercapacitors: aqueous electrolytes, organic electrolytes,

ionic liquids (ILs) and solid-state polymer electrolytes.[15]

Aqueous electrolytes like H2SO4, KOH, Na2SO4, and NH4Cl aqueous solutions can provide a

higher ionic concentration and lower resistance. However, the big disadvantage of aqueous

electrolyte is the small voltage window of 1.2 V, much lower than those of organic electrolytes.

Organic electrolytes can provide a high voltage window of 3.5~4 V. Acetonitrile and PC are

the most commonly used organic solvents. Acetonitrile can dissolve larger amounts of salt than

other solvents, but suffers from environmental and toxic problems. PC-based electrolytes are

friendly to the environment and can offer a wide electrochemical window, a wide range of

operating temperature, as well as good conductivity. Besides, organic salts such as

tetraethylammonium tetrafluoroborate, tetraethylphosphonium tetrafluoroborate and

triethylmethylammonium tetrafluroroborate (TEMABF4) have also been used in organic

electrolytes for supercapacitors. Water content has to be kept below 3-5 ppm to ensure the

operating voltage window safe.

ILs can exist in liquid form at desired temperatures, thus they are solvent free. A high voltage

of 5 V can be achieved for ionic-liquid based supercapacitors. ILs mainly include imidazolium,

pyrrolidinium, as well as asymmetric aliphatic quaternary ammonium salts with anions such as

tetra fluoroborate, trifluoromethanesulfonate, bis(trifluoromethanesulfony)imide,

bis(fluorosulfony)imide or hexafluorophosphate.[85-87] Room temperature ILs are usually

quaternary ammonium salts such as tetralkylammonium [R4N]+, and cyclic amines such as


aromatic pyridinium, imidazolium and saturated piperidinium, pyrrolidinium. Low temperature

ILs are based on sulfonium [R2S]+ as well as phosphonium [R4P]

+ cations.[85]

Solid-state polymer electrolytes are fabricated by mixing the electrolyte solution (e.g., acid or

alkalis in water, Li salt in organic solution, and ionic liquid) into a specific polymer matrix (e.g.,

polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyvinylidene difluoride (PVDF)). The

maximum working voltage of solid-state polymer electrolyte is determined by the electrolyte

solution: < 1.2 V for aqueous electrolyte based electrolyte and > 2 V for organic solution or ionic

liquid-based electrolyte. Due to the existence of polymer component, the ionic conductivity of

solid-state polymer electrolyte is lower than its liquid counterpart.[88] There is no leakage issue

and bulky packaging can be eliminated. It can also function as a separator between electrodes

and bind two electrodes together into an integrated unit, which is beneficial in developing

flexible energy storage devices.

Equivalent circuit

The equivalent circuit for most battery-type energy storage systems involves a Faradaic

resistance (RF), which represents the potential dependence of the reciprocal of the rate of the

oxidation and reduction charge transfer process. RF is in parallel with a double layer capacitance

Cdl (here Cdl is the overall capacitance of the device). Some diffusion control may arise under

high-rate discharge or charge, in which case RF is in series with a so-called Warburg C-R

impedance element written as W.[2] For both supercapacitors and batteries, an electrolyte

resistance element RE, in series with the Faradaic impedance ZF, is usually necessary in order to

fully represent the charging or discharging behavior. In fact, RE plays a very important role in the

evaluation and performance of supercapacitors and LIBs for high-rate discharge applications,

and has an important influence on the ac impedance spectrum of the devices.[2] The complete

equivalent circuit model is shown as follows.

Figure 2. 12. Equivalent circuit model for a battery-type energy storage system (e.g., LIBs and supercapacitors).


Bipolar cell design

Most batteries and capacitors have been made with conventional ‘monopolar’ technology that

uses two electrodes per cell and then connects those cells in a series of metallic connectors

outside of the cells, as shown in Figure 2. 13a. This design results in ohmic losses of the

electrodes leading to unsymmetrical distribution of the current density during operation.

Furthermore, these grid and cell connections increase the total weight of the battery. Bipolar and

monoploar designs share the same chemistry, while in bipolar systems as shown in Figure 2. 13b,

the cells are stacked in a sandwich construction so that the negative electrode of one cell

becomes the positive electrode of the next cell, from where the definition of bipolar comes.[89]

The cells are separated from each other by the bipolar electrode, which allows each cell to be

operated in isolation from its neighbor. Stacking these cells next to one another allows the

potential of the battery/capacitor to be increased. Since the cell wall becomes the connection

element between cells, bipolar electrodes have a shorter current path and a larger surface area

compared to monopolar cells. This construction reduces the power loss that is normally caused

by the internal resistance of the cells. At each end of the stack, single electrodes act as the final

anode and cathode. The bipolar construction leads to reduced weight, higher power/energy

density with smaller container than conventional monopolar systems. But bipolar systems require

that the edges of the electrodes should be carefully sealed to the case in order to avoid leakage

current, which has been the challenge for the commercialization of bipolar ones.

Figure 2. 13. Schematic illustrations showing cell designs of (a) monopolar, connected with metallic wires and (b)

bipolar. Reproduced from Ref. [89].


2.2.3 Electrode materials

Carbon materials

Currently, the most widely used electrode materials for double-layer supercapacitors are carbon

based materials. Carbon materials exhibit true capacitive behavior and excellent electrochemical

stability upon repeated cycling but the overall specific capacitance is relatively low compared to

pseudocapacitive materials. As shown in Figure 2. 14, CV curves of carbon materials have good

rectangular shapes, and the galvanostatic charge-discharge profile has triangular symmetrical

shape, indicating good capacitive behaviors. The typical capacitance of carbon materials is 50-

200 F g1

in aqueous electrolytes, 30-100 F g1

in organic electrolytes and 20-70 F g1

in ionic

liquids.[1] Since the working principle depends mainly on double layer capacitance at the

interface between the electrode and electrolyte, the surface area accessible to the electrolyte ions

would determine the whole performance of electrodes. Thus the specific surface area, pore-size

distribution, pore shape and structure, electrical conductivity, and surface functionality are all

playing very important roles.

According to Conway’s work, three requirements should be satisfied for selecting

supercapacitor electrode materials: (1) high specific surface areas, (2) good intra-and

interparticle conductivity in porous matrices, and (3) good electrolyte accessibility to the

intrapore space of carbon materials.[2] High specific surface area carbon materials mainly

include activated carbon, carbon fibers, carbon aerogel, CNTs, graphene, carbide-derived carbon,

and carbon onions. Many methods have been investigated to increase the specific surface area,

including heat treatment, alkaline treatment, steam or CO2 activation, and plasma surface

treatment with NH3,[15] through which micropores and defects on the carbon surface can be

obtained. It is also of significant importance to pay attention to the pore size since not all the

micropores in the electrode are necessarily accessible to electrolyte ions. It is reported that pore

sizes smaller than 0.5 nm were not accessible to hydrated ions,[90, 91] and that even pores under

1 nm might be too small for organic electrolytes since the size of the solvated ions is large than 1

nm.[92]

In addition to high specific surface areas and appropriate pore sizes, surface functionalization

has also been considered as an effective way to improve the specific capacitance of carbon

materials.[15] It is believed that surface functional groups or heteroatoms can adsorb ions, thus


improving the hydrophilicity and leading to enhanced wettability. On the other hand, functional

groups on the surface of carbon materials can introduce Faradaic currents and pseudocapacitance,

and finally leading to a 5-10% increase in the overall capacitance. Commonly used surface

functional groups contain oxygen, nitrogen, boron, sulfur and phosphorus.[1]

Figure 2. 14. (a) Typical CV curves of an activated mesocarbon/CNTs compound electrode; (b) galvanostatic

charge-discharge curves of the mesocarbon/CNTs electrode at a current rate of 0.5 A g1

. Reproduced from Ref. [93],

Copyright 2010, Elsevier.

Transitional metal oxides

Transitional metal oxides such as RuO2, MnO2, Co3O4, NiO, and V2O5 have been extensively

studied in the past decades. Three requirements for transitional metal oxides as electrodes for

supercapacitors are: i, the oxide should be electronically conductive; ii, the metal ions have two

or more oxidation states that coexist over a continuous range with no phase changes involving

irreversible modifications of a 3-dimensional structure, and iii the protons can freely intercalate

into the oxide lattice upon reduction and out of the lattice upon oxidation, allowing facile

interconversion of O2

and OH.[15] The specific pseudocapacitance exceeds the double layer

capacitance of carbon materials, promoting interest in these systems. However, on the other hand,

since redox reactions are involved, pseudo-capacitors often suffer from less stability during

cycling.

Ruthenium oxide is the first studied transitional metal oxide as electrode for supercapacitors

since it has good conductivity and three distinct oxidation states accessible within 1.2 V. It can

be described as a fast, reversible electron transfer together with an electron-adsorption of protons


on the surface of RuO2 materials, according to the following equation where Ru oxidation states

can change from Ru2+

up to Ru4+

:[2, 16]

xx(OH)RuOxexHRuO

22 (2. 8)

here 0x2. The continuous change of x during proton insertion or desertion occurs over a

window of 1.2 V and leads to a capacitive behaviors with ion adsorption following a Frumkin-

type isotherm.[2] Therefore the CV curve of RuO2 also has a rectangular shape with a mirror-

image symmetry, like EDLCs, similar to the behaviors of MnO2 as shown in Figure 2. 15.

Specific capacitance of more than 600 F g1

for RuO2 has been reported, but the high cost has

limited its application.

Less expensive oxides of iron, vanadium, nickel, manganese and cobalt have been tested.

Among all the oxides, manganese oxide shows the highest capacitance (theoretical specific

capacitance of 1370 F g1

). The fast, reversible successive surface redox reactions give rise to a

rectangular CV curve, as shown in Figure 2. 15.

Figure 2. 15. A CV curve of a MnO2 electrode in aqueous electrolyte (0.1 M K2SO4) shows the successive multiple

surface redox reactions leading to the pseudo-capacitive charge storage mechanism. The sequence of consistant

redox reaction between Mn3+

and Mn4+

. Reproduced from Ref.[16], copyright 2008 Nature, Macmillan Publishers

Limited.


The charge storage mechanism of manganese oxide is based on surface adsorption of

electrolyte cations C+ (K

+, Na

+…) as well as proton incorporation according to the following

equation: [16]

yxHMnOOCy)e(xyHxCMnO

2 (2. 9)

Manganese oxides also face several challenges: (i) Dissolution problem. Owing to the partial

dissolution of MnO2 in the electrolyte during cycling, they suffer from capacitance degradation.

The reason is that Mn2O3 or MnOOH can be converted into MnO2 and Mn2+

, the latter can be

dissolved in solution. To solve this problem, new electrolyte salts have been developed to avoid

forming acidic species in the solution. Another way is to coat a protective conducting polymer.

(ii) Low specific surface area and poor electronic/ionic conductivity. To solve these challenges,

effective ways would be developing nanostructured MnO2 materials, and probably introducing

other composite elements into them at the same time.[94] To enhance the electrical conductivity,

doping manganese oxides with ruthenium, gold, nickel, activated carbon, CNTs, graphite and

conducting polymers have been used.[15, 94]

Conducting polymers

Conducting polymers store capacitance through Faradaic redox reactions. They can be positively

or negatively charged with ion insertion (redox reaction) in the polymer matrix to balance the

injected charge.[15] The redox reactions in the conducting polymers occur only on the surface.

Since no structure alterations are involved upon charge/discharge reactions such as phase

changes, the processes are highly reversible.

The redox reactions are also termed as ‘doping’. The positively charged polymers, introduced

by oxidation on the repeating units of polymer chains, are termed as ‘p-doped’, while negatively

charged polymers generated by reduction are termed as ‘n-doped’. The potentials of these doping

processes are determined by the electronic state of π electrons.

The widely utilized conducting polymers for supercapacitors are PANI, PPy, PTh, poly (3,4-

ethylene-dioxythiophene) (PEDOT) and their derivatives.[15] PANI and PPy, which can only be

p-doped, are often used as cathode materials, because their n-doping potentials are much lower

than the reduction potential of common electrolyte solutions. PEDOT has a high stability, which

can be easily deposited as thin films. They can only work within a strict potential window,


otherwise the polymer may be degraded at too positive potentials, and switched to an insulating

state (un-doped state) at too negative potentials.

However, swelling and shrinking of conducting polymers may occur during the

intercalation/deintercalation process, leading to the mechanical degradation and fading

electrochemical performance during cycling. Thus the conducting polymer based supercapacitors

often remarkably degrade under less than a thousand cycles. To mitigate the challenge, many

methods have been investigates. (i) Improving the polymers’ structures and morphologies.

Nanostructured polymers provide a relatively short diffusion length to enhance the utilization of

electrode materials. (ii) Hybridizing supercapacitors. Since polymers in the n-doped state have

poorer stability than those in the p-doped state, replacing an n-doped polymer at the negative

electrode by carbon gives a better performance. (iii) Fabricating composite electrode materials.

Composite conducting polymers can improve the chain structure, conductivity, and mechanical

stability. Incorporating carbon into polymers is considered to be an effective solution to improve

the mechanical and electrochemical properties.[15] A PANI/CNT composite electrode could

even offer a high specific capacitance of 1030 F g1

in 1 M H2SO4 within a potential window of -

0.2~0.7 V at a current density of 5.9 A g1

.[95]

2.2.4 Flexible micro-supercapacitors

The idea of flexible electronics has already appeared for quite a time. Figure 2. 16 gives the

illustrations of various flexible mechanical and electrical devices. Unconventional burgeoning

consumer electronics have opened a new prospect of future wearable electronics such as smart

skins, human friendly implantable sensors, and stretchable circuitries.[96] A smart skin may

potentially provide a solution for on-body sensing that can monitor physiological signals and

healthcare data of human bodies while supporting people in various situations and activities.[97]

Conductive threads can be woven into fabrics, which are so called e-textiles. For example, a

medical-monitoring shirt integrated with light-emitting diodes, which could become

lifesavers,[98] not to mention electronics which can be implanted into human body to attach on

certain organs for special treatment. Wearable electronics has stimulated the developing and

designing of multifunctional electronic components including transistors, displays, and energy

harvesting, conversion and storage devices on a single textile substrate to achieve a fully self-


powered and self-sustaining integrated system. Thus high performance energy storage devices

with features of being small, thin, lightweight, and wearable are in great demand.

Flexible micro-supercapacitors have attracted extensive attention because they are capable of

providing higher power density, easier to be integrated with other components, and safer

compared to other energy storage sources. The challenge is to transform the device configuration

from being rigid and boxy to being flexible and thin while still maintaining good electrochemical

performance, reliability and integration.

Figure 2. 16. The schematic illustrations of various flexible mechanical and electrical sensors: (a) E-skins; (b)

Wearable and skin-attachable sensors; (c) Implantable components for in vivo diagnostics; (d) Advanced sensors

with additional functionalities, reprinted from Ref.[96].

The most commonly seen configuration of such flexible energy storage devices is based on a

solid planar structure, such as carbon cloth,[99] paper,[13, 100] Teflon plate, metal substrate,

poly-demethylsiloxane (PDMS), scotch tape, polyethylene terephthalate (PET).[101] Recently

energy storage devices in the form of fibers,[102-104] yarns[105-107] or wires,[108] which are

feasible to be woven into textiles or other similar structures, have also attracted great interest.


2.3 Similarities and differences between LIBS and

supercapacitors for electrochemical energy storage

A supercapacitor corresponds to an equilibrium situation at all states of charge across the CV or

along the charging curve, demonstrated by its symmetric CV curve in Figure 2. 15. In contrast,

the electrochemical processes in a battery are rarely reversible in the above mentioned sense and

a substantially different range of potentials is required for oxidation of the active material

compared with that for its reduction. The CV curve is asymmetric and no mirror-image

appearance is manifested, as shown in Figure 2. 4b.[2] In comparison with rechargeable batteries,

supercapacitors can endure higher number of cycles, be charged and discharged a hundred times

faster and reach at least 20 years of useful life, and also work at temperatures below 0 °C.[109]

The overall comparisons of LIBs and supercapacitors are listed in the following table.

Table 2. 3 Overall comparison between LIBs and supercapacitors. summarized based on Ref. [2].

LIBs Supercapacitors

1. High energy density

2. Relatively poor power density, depending on kinetics

3. Ideally has single-valued free energies of components

4. Irreversibility is common. Recharge curve is not mirror image of

discharge curve, e.g., in CV. Poor cycle life due to degradation of

active materials

5. Has corresponding single-valued potential or sloped potential

plateau during charge and discharge

6. Response to linear scan of potential gives irreversible I vs. V

profile with nonconstant currents

7. Has significant temperature-dependent activation polarization

8. Electrolyte conductivity can decrease or increase on charging,

depending on chemistry of cell reactions

9. Can be constructed in bipolar configuration

10. Temperature range:0-60 ºC

Limited energy density

High power density

Has continuous variation of free energy with degree

of conversion of materials or extent of charge

Excellent reversibility is common (cycle life > 100,

000). CV curves are symmetric.

Has corresponding continuous variation of potential

during charge and discharge

Response to linear scan of potential roughly gives

constant charging current profile but with some

dependence on materials

Has little or no activation polarization

Electrolyte conductivity can diminish on charging

due to ion adsorption

Can be constructed in bipolar configuration

-20~60 ºC

32 Chapter 3. Experimental methods

3 Experimental methods

This chapter discusses experimental methods and techniques: firstly, the deposition method to

prepare electrodes for LIBs and supercapacitors; secondly, the detailed fabrication process of a

single rolled-up Si battery; thirdly, the detailed fabrication of flexible supercapacitors; fourthly,

the currently used electrochemical measurement techniques including cyclic voltammetry (CV),

galvanostatic charge/discharge, potential step chronoamperometry (PSCA) and the

electrochemical impedance spectroscopy (EIS); and lastly, the characterization method for

materials including scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray

photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and Raman spectroscopy.

3.1 Deposition methods

3.1.1 Lithography

The purpose of lithography is to transfer patterns from a pre-fabricated photomask to other

photosensitive materials on a substrate. Optical lithography through UV-light exposure is the

most commonly used technique, including contact printing, proximity printing and projection

printing.[110] In addition, X-ray and electron beam lithography are also in use. The

photosensitive materials, named photoresist, consist of polymers whose chemical solubility can

be tuned after a certain light exposure. Generally, photoresists can be separated into positive and

negative ones, which are defined as follows: the exposed area of positive photoresists will be

removed by a chemical developer solution, while the unexposed area of negative ones is

removed. Well-defined structures can be precisely transferred to the substrate by specific

handling of the photosensitive layer and the substrate. Herein, a mask aligner MA 56 (SÜSS

MicroTec AG, Germany) working in contact mode was used.

In this work, a glass substrate was firstly cleaned with acetone and isopropanol, blown dry by

nitrogen, afterwards treated by oxygen plasma for 5 min. AZ-5214E (Microchemicals GmbH,

Germany) is actually a positive photoresist, but it enables the reversal of structures after

changing its solubility by appropriate thermochemical modification, resulting in a sharp undercut

like a high resolution negative resist. Therefore it is used in multistep lift-off procedures to create

Chapter 3. Experimental methods 33

patterns. To enhance the adhesion of the resist on a substrate, another additional adhesive layer

(TI Prime) is necessary before use, specifically on a glass substrate. The processing parameters

are as follows. TI prime: spin-coating at 3500 rpm (revolutions per minute) for 35 s, baking at

120 ºC hotplate for 2 min. AZ-5214E: spin-coating at 4000 rpm for 35 s, baking at 90 ºC hotplate

for 5 min. Then the glass substrate was exposed with UV-light for 2 s through photomask and

once again baked at 120 ºC hotplate for 2 min. The substrate was flood exposed to UV-light for

30 s, developed in an AZ-726 developer for 45 s~1 min, cleaned by deionized water, and finally

blown dry by N2.

3.1.2 Electron beam evaporation

One of the most popular techniques for fabricating thin films is electron beam evaporation,

which enables the deposition of almost all materials. It is a physical vapour deposition (PVD) in

which a source material is bombarded with an electron beam released from a charged tungsten

filament under high vacuum. Atoms from the source material is transformed into gaseous phase

and condensed on the surface of a substrate. Normally no chemical reactions are involved during

depositions, accompanied with a relatively low temperature on the substrate, thus resulting in

stable depositions of source materials. The thickness and deposition rate can be monitored by a

quartz crystal microbalance (QCM). Figure 3.1 shows a process flow example for rolling-up

nanomembranes. The substrate with patterned photoresist is transferred to the chamber of

electron beam evaporator, and deposited with a tilted angle in order to create an edge window for

the subsequent etching of photoresist, which serves as the starting point of the rolling process.

However, it is noteworthy that the resident films exist on the substrate all around the rolled-up

tubes. In order to roll up a single tube, multistep lithography is necessary to remove the resident

films. In this work, an electron beam evaporator BOC Edwards FL400, Germany was used to

deposit thin films. The details of rolling-up a single tube on chip are described in Chapter 4.


Figure 3. 1. Process flow for positioning rolled-up nanomembranes. a) Top and cross-section view of patterned

photoresist layer on a substrate; b) Schematic diagram of the tilted deposition method exploiting the ballistic shadow

effect; c) SEM image of rolled-up Ti/Au nanomembranes fabricated according to (b). Cited from Ref. [5].

3.2. Rolled-up nanotechnology

Rolled-up nanotechnology (strain engineering) was firstly introduced in 2001 by Schmidt and

Eberl in Nature.[4] By selectively etching a sacrificial layer (either inorganic or organic film), a

thin solid film can be released from a substrate with an internal strain gradient, which enables the

rolling up of the film. A scheme showing the rolling-up procedure is depicted in Figure 3. 2.

Three major parameters are influencing the strain state: (i) difference in the thermal expansion

between sacrificial layers and deposited films controlled by the substrate temperature, (ii)

deposition rate, and (iii) stress evolution during deposition.[5] By choosing appropriate

parameters, these 2D nanofilms can be rearranged into various 3D micro-/nanostructures

including tubes, helices, rings, wrinkles and other advanced micro-architectures with controlled

diameters, which find applications in micro/nano optics,[111] biology,[112-114] and

electronics[115, 116] as well as energy storage.[6, 8, 9, 117-119]

Figure 3. 2. A scheme showing the method to roll-up a tube. Strained layer can potentially be any kind of materials

with thicknesses in nanometer. Typical sacrificial layers can be GeOx, SiO2, photoresist, polyacrylamide, etc., each

of which can be etched with a specific solution.


Various materials with rolled-up tubular structures for advanced LIB anodes have been

developed, such as RuO2/C,[6] Ge/Ti,[8] Si/C,[9] GeO2,[117] SnO2/Cu,[119] and

SiOx/SiOy,[120] all of which have shown enhanced performance (e.g., capacity and cycling

stability) due to the fast ionic transport and powerful strain accommodation benefiting from the

unique micro/nano-hierarchical structures.

3.3 Electrochemical measurements

3.3.1 Cyclic voltammetry

CV is a widely utilized voltammetric measurement for electrochemical reactions involved

experiments, since it gives evident and intuitive descriptions of redox reactions. In a CV

measurement, the potential of working electrode is scanned at a certain rate to a set potential and

then scanned inversely. At the same time, the current response is recorded and plotted vs. the

applied potential. CV peaks occur because the current increases as the potential reaches the

redox potential of the reactant, but then falls off when the concentration of the reactants is

depleted near the electrode surface. The slower the scan rate, with thinner electrodes and highly

oriented particles, the narrower and better resolved are the peaks. With very thin and highly

oriented electrodes at sufficiently low scan rates, the CV may reflect a behavior that is beyond

diffusion control, approaching the thermodynamic limit.[121] Therefore, CV behaviors are

closely related with the diffusion effects, and helpful to understand electrochemical kinetics.

According to Nernst equation, the reversibility can be determined by the potential difference of

the anodic and cathodic peaks. When the potential difference is < 58 mV, the redox system is

reversible. Reversibilities are very important for electrochemical energy storage systems. The

specific capacitance of the overall capacitor can be calculated from the CV according to the

following equation:[15]

vmU

IdU

mU

v

dUI

mU

Idt

mU

QC

***2**2**2**2

(3. 1)

C (F g1

or F cm3

) denotes the specific capacitance, Q is the total charge integrated from CV

curves. U is the potential window, m is mass (in case of volumetric capacitance, here is the


volume V), v is the scan rate of CV measurement. In this work, a Zahner IM 6 was used to carry

out CV measurement.

3.3.2 Galvanostatic charge/discharge

Galvanostatic charge/discharge was carried out by inputting a constant current into the energy

storage device, and monitoring the voltage and the capacity. Various current densities could be

input. In this work, Arbin BT2000 system was used to perform the galvanostatic

charge/discharge measurement. Representative discharge-charge voltage profiles of a 2 m Si

film anode at various current densities are shown in Figure 3. 3, where plateaus indicate the

alloy/dealloy process of Si and Li.

Figure 3. 3. Representative discharge-charge voltage profiles of a 2 m Si film anode at various current densities

from 0.15 mA cm2

to 1.05 mA cm2

. Published in Ref. [122].

3.3.3 Potential step chronoamperometry

PSCA is an electrochemical technique in which the potential of the working electrode is stepped

and the resulting current from faradic processes occurring at the electrode (caused by the

potential step) is monitored as a function of time. In this work, small-amplitude of potential

perturbation was applied to the electrochemical system using Zahner IM6 to investigate the

different diffusion behaviors of the Si tube and planar film anode. In general, a PSCA

experiment is carried out for a redox reaction:[123]


ReOf

b

k

k (3. 2)

can be treated by invoking the current-potential characteristic:[123]

](O,t)eC,t)e([CFAkI )E)f(E(

R

)Ef(E

O

'' 00 10 0 (3. 3)

in conjunction with Fick’s laws, which can give the time-dependent surface concentrations. In

short time region, the current-time response is as follows:[123]

2/12/1

2/1

)(t

CnFADtI OO

(3. 4)

which is known as the Cottrell equation, and independent of geometries of the electrode.

Therefore, the diffusion coefficient of ions can be obtained. The details will be discussed in

Chapter 4.

3.3.4 Electrochemical impedance spectroscopy

EIS is realized by applying a single-frequency voltage or current to an electrochemical system

and measuring the phase shift and amplitude (Bode plot), or real and imaginary parts (Nyquist

plot), of the resulting current at that frequency using either analog circuit or fast Fourier

transform (FFT) analysis.[124] It is a useful and convenient way to detect both the intrinsic

properties and external stimulus, which influence the conductivity of an electrochemical system.

The frequency response to a small-amplitude ac signal in a frequency range of 10-4

and >106 Hz

can be measured and analyzed. Here Zahner IM 6 was used to carry out EIS measurement. A

simplified analog circuit, including resistor and capacitor connected either in parallel or in series,

can be used to analyze a LIB or supercapacitor, such as Figure 2. 12. The following schematic

shows a Nyquist plot of an ideal capacitor and an electrochemical capacitor, both having the

same ESR (equivalent series resistance at 1 kHz). The ideal capacitor exhibits a vertical line,

while the electrochemical capacitor starts with a 45 º impedance line, approaching an almost

vertical line only at low frequencies.[3]


Figure 3. 4. Schematic representation of the Nyquist impedance plot of an ideal capacitor (vertical thin line) and an

electrochemical capacitor with porous electrodes (thick line). Cited from Ref. [3].

3.4 Characterization methods

3.4.1 Scanning electron microscopy

SEM enables the investigation of materials with micro/nano-structured surfaces. A focused beam

of electrons is scanned over a rectangular raster area onto a sample, and interacts with atoms in

the sample, producing various detectable signals which contain the information about the surface

topography and composition. These signals include secondary electrons (SEs), back-scattered

electrons (BSEs), characteristic X-rays, transmitted electron, and so on. Among them, SEs are

most commonly used to detect the morphology of a sample. A number of electrons at each raster

point can be detected and converted into an intensity map. BSEs are often used in analytical

SEM along with the spectra made from the characteristic X-rays, since the intensity of the

inelastic scattered BSE is strongly related to the atomic number of the element. SEs have lower

energies than BSEs and are given off from the substrate near the sample as a side effect. In this

work, Zeiss DSM982 and NVision40 were employed for SEM characterizations. Prior to the

SEM characterization, a single Si tube was coated with 5 nm of Au by electron beam evaporator

for a good surface conductivity.


3.4.2 X-ray diffraction

XRD detects the crystal structure, chemical compositions, the average spacing between layers or

rows of atoms, as well as the size, shape and internal stress of a material. X-ray was discovered

in 1895 by Röntgen and XRD was found in crystals in 1912 by Laue. Then the Braggs succeeded

in analyzing the crystal structure of NaCl using XRD. X-ray is a form of electromagnetic

radiation, with a wavelength between 0.01 to 10 nm and energies in the range of 100 eV ~ 100

keV. The principle is based on Thomson scattering (elastic) of X-rays. In an X-ray diffraction

measurement, a material is bombarded with X-rays, producing a diffraction pattern of regularly

spaced spots known as reflections. The 2D images taken at different rotations are converted into

a three-dimensional model of the density of electrons within the crystal using the mathematic

method of Fourier transforms, combined with chemical data known for this sample. In this work,

an X-ray diffraction (PANalyticalX'Pert PRO Diffraction, Co-Ka radiation, reflection geometry)

was used. The indexing of the reflections was carried out using the PDF-4+2010 database of the

International Center for Diffraction Data (ICDD).

Figure 3. 5. Schematic showing the working principle of XRD. Reprinted from [125].

3.4.3 X-ray photoelectron spectroscopy

XPS is used to investigate the surface (maximum ~10 nm) of a material and can be used to

determine the valence state of elements. In this work, SPECS PHOIBOS 100 was used. In XPS,

photons of low energy (~1.5 keV) excite the core electrons above the Fermi level. The energy

spectrum of the emitted photoelectron is determined by means of an electron energy

spectrometer. X-ray irradiates the sample in ultra-high vacuum environment, and atoms near the

sample surface emit electrons (photoelectrons) either from the core-level or the valance band.

The energy of photo-emitted electrons is related to the electronic structure of the atoms or

molecules from which they are excited. The number of characteristic photo-emitted electrons is

proportional to the concentration of the corresponding type of atoms in the material. The


working principle of XPS is shown in Figure 3. 6. The kinetic energy of electrons emerging from

the surface plus the binding energy of the electrons equals the incident energy of X-ray.

Therefore the binding energy of atoms can be determined by measuring the kinetic energy of

emitted electrons.[126]

Figure 3. 6. A Schematic diagram shows the working principle of XPS. Cited from Ref. [126].

3.4.4 Atomic force microscopy

AFM gives 3D information of a surface in the nanometer scale including the roughness, depth,

and morphology. The working principle is designed on the basis of measuring forces between the

tip and sample.[127] The tip is mounted at the end of a flexible cantilever. The force is measured

by the spring constant of the cantilever and the tip-sample distance, and described by Hook’s law.

If the tip is quite far away from the sample surface, forces hardly exist. When the tip comes

closer to the surface, the attractive Van der Waals force is dominant. Even closer or nearly in

contact with the surface, the repulsive Van der Waals will dominate in the system. In this work, a

Nanoscope III SPM (Digital Instruments) was used.

3.4.5 Raman spectroscopy

In this work, Renishaw with 442 nm wavelength was used to carry out Raman spectroscopy.

Raman effects, observed in practice in 1928 by Sir C. V. Raman, are used to observe vibrational,

rotational, and other low-frequency modes in a system. [128] The working principle is based on

inelastic (Raman) scattering of monochromatic light, usually from a laser in the visible, near

infrared, or near ultraviolet range. An incident laser photon interacts with a molecule (solid,


liquid or gaseous), resulting in the molecule in a virtual energy state for short time before an

inelastically scattered photon is produced. The inelastically scattered photon can be of higher

(anti-Stokes) or lower (Stokes) energy than the incoming photon. To keep balanced, the

vibrational state of the molecule will be shifted down or up. The shift in energy gives

information about the vibrational modes in the system. Rayleigh scattering is an example of

elastic scattering, which is of the same frequency as the incoming electromagnetic radiation. In a

Raman spectroscopy measurement, elastic Rayleigh scattering is filtered out while the rest of the

collected light is dispersed onto a detector by either a notch filter or a band pass filter. A

schematic depicting the shift of energy states in two kinds of scattering is shown as follows:[129]

Figure 3. 7. A schematic energy-level diagram indicating the states involved in Raman signal. The line thickness is

roughly proportional to the signal strength from the different transitions. Cited from Ref. [129].

42 Chapter 4. A Single Rolled-Up Si Tube Battery

4. A single rolled-up Si tube micro-battery

4.1 Introduction

This chapter is based on the publication of Adv. Mater. 2014 (DOI: 10.1002/adma.201402484),

titled “A Single Rolled-Up Si Tube Battery for Study of Electrochemical Kinetics, Electrical

Conductivity and Structural Integrity”.

Silicon based anodes for LIBs have been the focus of intensive research interests, primarily

because silicon exhibits a very high theoretical specific capacity (4200 mAh g1

for Li22Si5 at

415 °C)[41] and is among the most promising candidates that can replace graphite as anodes in

rechargeable LIBs. At room temperature, Li15Si4 (or a-Li3.75Si) is the highest lithiated phase

achievable or the lithiation of silicon, corresponding to a capacity of 3579 mAh g1

.[42-46] Upon

repeated alloying-dealloying, a huge volume change of Si (280%)[44, 48] also accompanies the

lithiation-delithiation process, which leads to the loss of contact or pulverization of the

electrodes. An effective approach to enhance the cycling performance is to utilize nanostructured

Si and their composites with carbon materials, such as Si nanoparticles,[49] Si

nanowires/nanorods,[47, 50] Si nanotubes[51] and Si nanospheres.[52] Designing and

fabricating micro/nanoscale hybrid materials to take advantage and restrain shortcomings of

them is an important approach to develop high performance LIBs.[1]

Rolled-up nanotechnology[4, 5, 130] offers an advanced strategy to deterministically rearrange

2D nanomembranes into 3D micro/nano-hierarchical tubes, helices, rings, wrinkles and other

advanced architectures for micro/nano optics,[111] biology,[112-114] and electronics[115, 116]

as well as energy storage applications.[6, 8, 9, 117-119] In our group, we have reported on

various materials with rolled-up tubular structures for advanced LIB anodes, such as RuO2,[6]

Ge/Ti,[8] Si/C,[9] GeO2,[117] SnO2/Cu,[119] and SiOx/SiOy,[120] all of which have shown

enhanced performance (e.g., capacity and cycling stability) due to the fast ionic transport and

powerful strain accommodation benefited from the unique hollow micro/nano-hierarchical

structure. Despite this, a comprehensive understanding of the correlation between the electrodes’

tubular structure, electrical/ionic conductivity and the electrochemical kinetics as well as the

performance is still missing. However, it is of a great challenge to carry out these studies with

Chapter 4. A Single Rolled-Up Si Tube Battery 43

bulk batteries, since they are often composed of electrodes mixed with conductive additives and

polymer binders, which introduce significant uncertainties into any quantitative electrochemical

kinetic studies. In order to eliminate these uncertainties, the investigation of a single unit of

active material (herein a single rolled-up tube) should be an attractive approach.

Considerable efforts have been dedicated to developing such platforms using single

micro/nanoscale active materials with well-defined geometry as independent anode or cathode in

LIBs, which enables direct diagnosis of the electrochemical behaviors and

observation/characterization of the structural/compositional changes of active materials.

Uchida’s group[83, 84] investigated the kinetics of Li+ extraction/insertion by measuring the

current/potential behaviors of a single particle (graphite and LiMn2O4), in which a

microelectrode was attached to the particle to realize electrical contact. Mai and co-workers[81]

have developed an electrode platform with the ability to in-situ record the electrical transport and

the structural evolution of single silicon nanowire. Huang and coworkers[56] have reported a

pioneering work using in-situ TEM to observe the lithiation of a single SnO2 nanowire during

electrochemical charging and found that a reaction front containing a high density of dislocations

electrochemically drives the solid-state amorphization of the nanowire. Wang and co-

workers[58] reported a two-phase process of electrochemical lithiation in amorphous Si by using

in-situ TEM, which further leads to a inhomogeneous and discontinuous volume expansion.

MacDowell and Cui[80] have investigated the electronic properties and structural changes in a

single Si nanowire.

In this study, a lab-on-chip electrochemical device platform is presented for probing the

electrochemical kinetics, electrical properties and lithium driven structural changes of a single

silicon rolled-up tube as an anode in LIBs. The chemical diffusion coefficient (D) of the Li into

a Si tube is determined and compared to the case of a planar film. The electrical conductivity of

the single tube at various potentials is also investigated since it is closely related to the

magnitude of the battery’s driving force.[14] The lithium-driven structural changes clearly show

the formation of wrinkles due to strain-induced local deformations during repeated

lithiation/delithiation, confirming that the tubular structure possesses powerful strain

accommodation properties.


4.2 Fabrication of a LIB with a single Si rolled-up tube as

anode

4.2.1 A single rolled-up Si tube

The detailed fabrication procedures of a single rolled-up Si tube are shown in Figure 4. 1. In

total, three steps of lithographies were carried out. All patterns were fabricated using the lift-off

photoresist AZ-5214E by the photolithography. And all the thin films were deposited vertically

without tilting the substrate.

The first lithography patterned a blank square on a glass substrate utilizing a MA56 mask

aligner in contact mode. A 20 nm-Ge layer was deposited onto the square by electron beam

evaporation, which would work as a sacrificial layer for rolling-up a single tube.

The second lithography gave a patterned photoresist sealing three edges of the Ge layer and the

area around it, which later would offer three opened windows during the tube rolling process and

remove all the residual films around the square. The sample was treated in oxygen plasma (10

min/50 W) and then baked overnight on a hotplate at 60°C to oxidize the Ge layer into GeOx for

easy etching in water. Afterwards, the strained silicon layer (45 nm) was deposited.

The last lithography patterned the structures for the deposition of Ti (200 nm) electrical

contacts of the tube. The sample was immersed into deionized H2O for two hours and a single

silicon tube was rolled up with a diameter of around 30 nm and a length of 480 μm. Finally, the

tube was dried in critical point dryer (CPD) to avoid the collapse of the microtube caused by

surface tension forces during solvent evaporation. At a low temperature of 10 ºC, the initial

solvent (acetone) in the CPD chamber was replaced by liquid CO2 until the chamber pressure

reached up to 50 bar. When the pressure was stable and no acetone was left in the chamber, the

CPD chamber was heated up to 39-40 ºC to volatilize CO2 beyond the supercritical point.


Figure 4. 1. A schematic depicting procedures for rolling up a single Si tube (1) patterning and deposition of Ge as

sacrificial layer, (2) patterning of 2nd

photoresist and oxidizing Ge layer into GeOx by oxygen plasma treatment, (3)

deposition of Si layer, (4) deposition of Ti contacts, (5) rolling the tube by removing GeOx, (6) a real optical

micrograph of a single rolled-up Si tube.

4.2.2 Assembly of a micro-battery

Then a micro-battery would be assembled using the Si tube as anode, and a strip of lithium foil

were used as counter electrode. The polydimethylsiloxane (PDMS) chamber was used as an

electrolytic cell. A PDMS mixture solution (1 : 10, Sylgard 184 Silicone Elastomer KIT, Dow

Corning, MI, USA) was poured onto a blank Si wafer (three iron cylinders with 5.5 mm in height

and 5 mm in diameter as dies for the chamber formation) and cured for 20 min at 100 ºC. After

peeling off the PDMS from the Si wafer, a pinhole was made in the center of one of the cylinder-

shaped chambers with a cutting needle, for afterwards electrolyte filling and lithium foil

positioning. The glass and PDMS were treated by oxygen plasma for 30 s at 30 W, and these two

components were carefully brought together with the single tube at the center of one pinhole-free

cylinder chamber, ensuring that the distance between the tube and lithium foil is 5mm. Extra

PDMS solution was brushed along the attaching edges and cured to enhance the binding

strength. Then the device was transferred into an Ar-filled glove box (H2O < 0.1 ppm, O2 < 0.1


ppm, MBraun, Germany). 1 M LiPF6 (EC, DEC and DMC (1:1:1 by weight, Merck)) was then

filled into the device chamber as electrolyte. Li foil was placed through the pinhole as counter

electrode, and afterwards extra PDMS solution was used to seal the pinhole. The schematic of an

encapsulated battery is shown in Figure 4. 2. Li foil has a much larger surface area than the Si

rolled-up tube, and the influence of Li foil during electrochemical test is little. Therefore, the

electrochemical responses are mainly controlled by the rolled-up Si tube.

Figure 4. 2. A schematic illustration of an encapsulated Li-ion battery with a single silicon tube as anode.

4.3 Results and discussion

4.3.1 Characterization of rolled-up Si tube

Figure 4. 3a shows an optical micrograph of the single rolled-up Si tube with a diameter of

around 30 m, to which three Ti contacts are made with one in the middle of the tube and two at

its two ends. The rolled-up Si tube was characterized by Raman spectroscopy with 442 nm

wavelength. The Raman spectrum of the single Si tube (Figure 4. 3b) shows a broad peak at 475

cm1

, which is a characteristic feature of the amorphous silicon vibration modes due to the first

order transverse optical phonon.[131]


Figure 4. 3. (a) Optical micrograph of a single rolled-up silicon tube contacted with Ti strip lines. (b) Raman

spectrum of a single silicon tube.

4.3.2 Electrochemical properties of a single rolled-up Si tube

CV and PSCA were carried out using a Zahner IM6 electrochemical workstation. The

lithiation/delithiation of the silicon tube was characterized by CV at a scan rate of 100 V s1

for

the first three cycles, as shown in Figure 4. 4a and b.

In the first cycle, three main cathodic peaks (a, b, c) occur at 0.173, 0.089 and 0 V during

discharging, while two main anodic peaks (d, e) occur at 0.267 and 0.450 V during charging. A

small suppressed cathodic peak (indicated by the arrow) at 0.288 V is identified as well, which

corresponds to the irreversible formation of a SEI layer. The first cathodic peak a at 0.173 V is

extremely sharp and well-distinguishable, indicating a two-phase region.[42] Wang et al.

confirmed that the two-phase region includes amorphous Si and an amorphous LixSi (x~2.5)

product.[58] This sharp peak disappears in subsequent cycles and a broad peak emerges at

almost the same potential. The second cathodic peak b at 0.089 V has a broad shape, indicating a

single-phase region. This peak shows a significant negative-shift in the subsequent cycles due to

the kinetic effects.[123] To clarify the third cathodic peak c at 0 V, a control sample with only Ti

contacts on the glass substrate was examined and the corresponding CV results (Figure 4. 4d)

show that this peak does stem from the background current. The two anodic peaks d and e both

have broad shapes, which are typical for delithiation of amorphous Si,[42, 44, 132, 133] and

show positive-shifts in the subsequent cycles.


As a reference, the CV behaviors of the planar Si film are shown in Figure 4. 4c. During the

first discharging, only one sluggish cathodic peak a’ is observed with a starting potential at 0.212

V and a peak potential at 0.156 V, which is a broad peak of the two overlapped sharp peaks (a

and b in Figure 4. 4b). As mentioned before, the peak b’ at 0 V is mainly contributed by the

background current. The irreversible reaction peak corresponding to the SEI formation occurs at

around 0.310 V (indicated by the arrow). During the first charging, the two broad anodic peaks

(c’ and d’) located at 0.325 and 0.520 V, respectively, are both positively shifted compared to the

tube’s anodic CV peaks d and e.

As already mentioned in Chapter 3, a CV peak occurs because the current increases as the

potential reaches the redox potential of the reactant, but then falls off when the concentration of

the reactants is depleted near the electrode surface. The slower the scan rate, with thinner

electrodes and highly oriented particles, the narrower and better resolved are the peaks. With

very thin and highly oriented electrodes at sufficiently low scan rates, the CV may reflect a

behavior that is beyond diffusion control, approaching the thermodynamic limit.[121] Therefore

the emergence of the sharp, well-resolved peak a reflects that the diffusion of Li ions is fast

enough to maintain the alloying process with the Si tube, and the behavior is reaction controlled;

while the sluggish, overlapped peak a’ indicates the electrode’s behavior is diffusion controlled,

further leading to hysteretic kinetics.

In summary, compared to the planar film and literature results,[132, 134, 135] the current

response of the electrochemical reaction in a single tube is instant, well-distinguishable, and no

overlapping between different reactions occurs due to enhanced ionic conduction (larger

diffusion layer) and less kinetically involved polarization originating from the unique tubular

structure. These features enable the unique tubular structure to be further used as advanced

ultramicroelectrode for other electrochemical applications.


Figure 4. 4. CV curves of the first three cycles at a scan rate of 100 V s1

between a potential window of 1-0 V for

(a) a single Si rolled-up tube; (b) magnified part of the CV for a single Si rolled-up tube; and (c) an on-chip Si planar

film; (d) background current measured with bare Ti contacts on glass.

4.3.3 Chemical diffusion and electrical conductivity of a single rolled-up Si

tube

Diffusion properties of LIBs determine some of the key performance parameters, including the

charge/discharge rate, reversible capacity and cycling stability. Normally, it is very challenging

to determine D for composite electrodes due to their inhomogeneous potential distributions and

unknown electrode surface area. Therefore, a single rolled-up tube platform is established and

PSCA is performed to gain a better understanding of the different diffusion behaviors of Li ions

in the Si tube and planar film anode. The calculation of D from the PSCA measurements has

been described in detail, previously.[123] Briefly, the potential is stepped and the response

current from faradic processes occurring at the electrode is monitored as a function of time.

Figure 4. 5a shows a typical current response when the potential of the Si tube was stepped from


0.200 V to 0.180 V vs. Li/Li. A clear short-time current response region can be identified, in

which the current decreases rapidly. This short-time current response can be treated as the semi-

infinite diffusion, and described by the Cottrell equation in Chapter 3.3.3:

2/12/1

2/1

)(t

CnFADtI OO

(3.4)

n denotes the electron number involved in the reaction, F Faradaic constant, A the surface area,

and

OC the bulk concentration of Li

ions. For the ease of calculation, the Cottrell curve was

replotted as I vs. t1/2

in Figure 4. 5b. A straight line was observed between 5 and 27 s, and the

potential-dependent diffusion coefficient D can thus be obtained as shown in Figure 4. 5c.

Figure 4. 5. (a) The current-time response (b) Cottrell plot of the current-time response of a single rolled-up Si tube

after the potential was stepped from 0.200 to 0.180 V vs. Li/Li. (c) .Chemical diffusion coefficient (D) of Li ions in

Si tube and planar film under various potentials during the first discharge.


A minimum D emerges around 0.4 V caused by the thickened diffusion path after the

formation of the SEI layer. A maximum D occurs around 0.2 V corresponding to the sharp CV

peak a (Figure 4. 4b), which is probably due to the increased contact area between the electrode

and electrolyte because the fast reaction between Li and Si introduces a large volume expansion

as well as the formation of wrinkles and small particles on the surface (see SEM images in

Figure 4. 7c and d). In addition, from the thermodynamic point of view, the diffusion coefficient

is largely affected by the boundaries or interfaces of the materials.[136] The two-phase region

occurs around 0.2 V, resulting in large amounts of defects and interfaces. It is believed that the

generated defects and interfaces would facilitate the diffusion of Li ions, thus a maximum D

occurs around 0.2 V. The maximum diffusion coefficient D of Li+ ions to a tube seems more

evident. But it is worth mentioning that D in a planar film is also increased from the minimum at

around 0.3 V to 0.2 V. Only the increasing amplitude is not as high as for the tube. Again, I

would like to attribute this feature to the three-dimensional structure, which enables the sufficient

contact of the electrolyte and electrode, furthermore the sufficient diffusion of Li ions to the

electrode. This leads directly to the well-resolved CV peaks of the tube. For a planar film, since

the diffusion is sluggish, the current responses, i.e., the CV peaks are therefore sluggish. Similar

features were also reported for amorphous Si films in literature.[137] Various values for the

diffusion coefficient of Li in amorphous Si have been reported both experimentally and

theoretically ranging from 1014

to 109

cm2 s1

with a typical value of 1012

cm2 s1

.[137-142] In

our work, the diffusion coefficient of Li in Si tube is around 10

11 cm

2 s

1, which is several

times higher than that in planar Si films under all potentials except at 0 V. The enhanced

diffusion of Li in a tubular structure as compared to the planar film provides clear explanation

for the well-resolved CV peaks.

Additionally, special attention should be paid to the cell time constant du

CR (u

R for

uncompensated resistance, d

C for double layer capacitance) in microelectrode applications, since

ten times of it gives rise to the minimum time which is needed for a new potential establishment

in potential step experiments. du

CR is proportional to the working electrode’s size, as described

in the following equation:[123]

4

0

0 d

du

CrCR (4. 1)


0r denotes the radius of a microelectrode, 0

dC the capacitance per area and the electrolyte

conductivity. Here, a typical value of 0

dC =20 F cm

2 is used,[123] a value of =0.003

1 cm

2

responsible for the 1 M LiPF6 electrolyte,[143] and 0

r =480 m is the size of the microelectrode,

resulting in a cell time constant of 14.4 s and a minimum measurement time of 144 s. The

small value indicates that a new potential can be stepped instantly for the microelectrode, which

confirms the applicability of the Cottrell equation to our case.

Figure 4. 6. (a) Typical I-V curves of the single Si tube under various lithiation/delithiation states; (b) electrical

conductivity at various discharge/charge states during the first cycle.

In addition to the ionic conduction, the electrical conductivity of the electrode also plays a key

role in determining the battery’s rate performance. The electrical conductivity was measured

using Ti contacts on the two ends of a fresh tube after each lithiation/delithiation step during

electrochemical charge and discharge cycling. A Keithley series 2612A source meter was used.

Before the measurement, each potential was held at least for one hour. Typical I-V curves at

various lithiation/delithiation states can be found in Figure 4. 6a and the calculated electrical

conductivities are shown in Figure 4. 6b. The lithiation of Si occurs below 0.3 V according to

CV results, while significant changes in electrical conductivity are observed below 0.25 V vs.

Li/Li. The maximum conductivity of 0.56 S cm

1 occurs around 0.12 V, which has been

attributed to the decomposition of the conductive alloy and the formation of the less-conductive

alloy.[135] Although the value is lower than that measured by Pollak et al. for a 100 nm

amorphous Si,[135] it is similar to that obtained by McDowell et al. for a 50 nm amorphous Si

film.[80] In addition, the volume expansion of the Si tube upon the insertion of lithium


contributes to the increase of the overall conductivity, since the cross section increases during the

expansion of the tube. Overall, the electrical conductivity of the Si tube during discharge is

higher than that during charge.

4.3.4 Structural observation of Si tube before/after cycling

The surface morphology of the tube was characterized by SEM, Zeiss DSM982. Figure 4. 7a-c

show the SEM images of the single Si tube before cycle (30.9 m in diameter, 480 m in

length), revealing a smooth surface. After three cycles, the tube exhibits a highly wrinkled

structure with apparently accommodating large volume strains introduced during electrochemical

processes (Figure 4. 7d and e), while the diameter has expanded to 53.4 m. Iwamura et al.

demonstrated that a wrinkled structure can also be formed on Si nanoparticles at early periods,

and when freezing the structure at the wrinkled state, nano-Si can be stably charged/discharged

for a long period and show excellent rate performance as well.[49] Considering this viewpoint,

the wrinkles introduced in this experiment would also offer the possibility for high power LIBs

with enhanced cyclability.

Figure 4. 7. SEM images of a tube (a-c) before cycling with various magnifications, (d and e) after three cycles of

lithiation/delithiation.


4.4 Conclusion

In conclusion, a lab-on-chip electrochemical device is demonstrated that allows for the study of

the lithiation and delithiation process of a single rolled-up Si tube, the determination of the

diffusion coefficient and electrical conductivity, as well as the characterization of structural

changes. CV curves of the Si tube exhibit sharp, better resolved peaks compared to that of a

planar Si film due to the enhanced diffusion effect in the unique tubular structure, which makes

the rolled-up tube a promising candidate for an ultramicroelectrode for electrochemical studies.

A maximum electrical conductivity occurs after the lithiation of the Si tube. After three cycles,

the tube exhibits a highly wrinkled structure due to the strain-induced local deformation, which

could be exploited to maintain a stable charge/discharge cycling. The single rolled-up tube

battery described here is promising for the fundamental research of voltammetry and

electrochemical processes and could also be used as local on-chip energy supply or for driving

ultra-compact autonomous microsystems.

Chapter 5. On chip, all solid-state and flexible micro-supercapacitors 55

5. On chip, all solid-state and flexible

micro-supercapacitors based on MnOx/Au

multilayers

5.1 Introduction

This chapter is based on the publication of Energy Environ. Sci. 2013, 6, 3218. titled “On chip,

all solid-state and flexible micro-supercapacitors with high performance based on MnOx/Au

multilayers”.

Energy storage components have become a bottleneck for the reduction of the size of

microelectronic devices. The recent rapid advance and eagerness of miniaturized, portable

consumer electronics stimulate the development of micro-scale power sources with high power

density, towards the trend of being small, thin, lightweight, flexible, and even wearable, to meet

the growing demands of modern society.[10-13] On the other hand, introducing small solid state

energy storage devices has stimulated significant research interest, which enables circuit

designers to place energy storage devices directly on a chip for load powering. However, it is

still a challenge to realize high performance flexible energy storage devices because they are

highly dependent on the electrical and mechanical properties of the electrode materials.

Supercapacitors, also named electrochemical capacitors, bridging the gap between high energy

batteries and high power conventional electrostatic capacitors, have attracted great attention.[2,

3, 144, 145] Two main working principles are established regarding supercapacitors. One is

electrochemical double layer charge storage through which carbon based materials can work.

Another type of principle is pseudo-capacitive charge storage, including rapid redox reactions at

the surface and bulk of the electrodes, which lies at the heart of conducting polymers- and

transition metal oxides-based supercapacitors.[2] MnO2-based pseudocapacitive electrodes have

been demonstrated as a promising solution to various energy applications because of its high

theoretical specific capacitance (1370 F g1

), environmentally friendly nature and low cost of

raw materials. However, it is difficult to achieve high theoretical capacitance with bare MnO2,

primarily limited by the poor electrical conductivity (105

-106

S cm1

)[94, 146-148]. Extensive

56 Chapter 5. On chip, all solid-state and flexible micro-supercapacitors

efforts have been dedicated to improve the capacitance through adjusting crystal forms (e.g.,

amorphous and -, - and -type crystalline MnO2 electrodes),[149-152] defect chemistry (e.g.,

Mn-Me mixed oxides, MnO2-polymer composite electrodes, MnO2-nanostructured carbon

composites),[99, 153-155] morphology (e.g., nanofibers, nanorods, nanosheets)[156-158] and

porosity[154, 159-161] of MnO2.

Moreover, mechanical issues such as low structural stability and flexibility as well as

electrochemical dissolution of active materials are also plaguing for MnO2-based electrodes,

resulting in degraded long-term electrochemical cyclability.[94] It is especially important to

enhance the structural stability and eliminate electrochemical dissolution of MnO2, when

concerned with thin-film electrodes. Intensive explorations have been done on thin-film or MnO2

coated electrodes through varieties of techniques, including sol-gel dip-coating,[162]

anodic/cathodic electrodeposition,[148, 163-165] electrophoresis,[166-168] and sputtering-

electrochemical-oxidation or direct sputtering deposition.[169-171] To the best of our

knowledge, however, no reports have been found for MnO2 thin-film supercapacitors deposited

with electron beam evaporation. Electron beam evaporation is a physical vapor deposition

method to grow thin films, in which certain source materials undergo decomposition in the vapor

phase, offering the opportunity to control the structural and stoichiometry of materials. The thin-

film deposited with electron beam evaporation possesses good adhesion to substrate, ensuring

long-term cycling stability of supercapacitors, which is compatible with the industrial-level

technologies to meet the application demand for advanced materials.

In this work, a new concept is introduced to fabricate on chip, all solid-state and flexible

micro-supercapacitors based on MnOx/Au multilayers, which is compatible with current

microelectronics. The micro-supercapacitor exhibits a maximum energy density of 1.75 mWh

cm3

and a maximum power density of 3.44 W cm3

, which are both much higher than the values

obtained for other solid-state supercapacitors. At a scan rate of 1 V s1

, a volumetric capacitance

of 32.8 F cm3

is obtained for MnOx/Au multilayers electrodes, which is much higher than the

bare MnOx electrode. EIS and the evolution complex capacitance confirm that the electrical

conductivity of MnOx is improved due to incorporation of gold, and a low relaxation time

constant around 5 ms is observed. The MnOx/Au multilayers micro-supercapacitor also shows

good long-term cycling stability, with a capacitance retention rate of 74.1% after a large cycling


number of 15 000 times. Compared with other supercapacitors, which are not portable, and

relatively bulky, the device demonstrated here allows fast and reliable applications in a portable

and smart fashion. Furthermore, the nature of the process allows the micro-supercapacitor to be

integrated with other micro-devices, to meet the need for micro-scale energy storage.

5.2 Fabrication of solid-state micro-supercapacitors

A photograph of the on chip, all solid-state and flexible micro-supercapacitors is shown in Figure

5. 1a. The symmetric micro-supercapacitor was equipped with 24 interdigital fingers (twelve

fingers as anodes and cathodes respectively), which were fabricated onto flexible PET substrate

by using lift-off photoresist AZ-5214E patterned by photolithography. A schematic diagram of

the micro-supercapacitor is shown in Figure 5. 1b. Electron beam evaporation is used to deposit

electrical contacts and active materials for electrodes. The electrical contacts were achieved by

depositing Cr/Au (5/50 nm) at a vacuum below 3×106

mbar. Anode and cathode were consist of

with a 50 nm-thick MnOx/Au multilayered film, stacked in the order of

MnOx/Au/MnOx/Au/MnOx (shown in Figure 5. 1b), with a thickness of 15 nm and 2.5 nm for

each MnOx and Au layer, respectively. It is worth mentioning that when MnOx was deposited,

the oxygen flux was kept at 2×104

mbar. To ensure the accuracy of the thickness for the

deposited films, a 200 nm film was first deposited, and the actual thickness was then measured

by the Profilometer Dektak XT (Brucker). Thus a calibrated tooling factor for the deposition of

each material was obtained, which was used in the following deposition processes. The source

material for MnOx deposition is 99.9% manganese (IV) oxides pieces from American elements,

USA.

For the solid electrolyte preparation, 3 mL H2SO4 was dropped into 30 mL deionized water

and then 3 g PVA powder was added. The whole solution was kept at 85 °C under vigorous

stirring until the solution became clear. The gel solution was dropped cast to the surface of the

microelectrodes. After the PVA-H2SO4 gel solidified, the preparation of the micro-

supercapacitor was finished.


Figure 5. 1. (a) Photograph of a real, flexible micro-supercapacitor on polyethylene terephthalate (PET) substrate. (b)

Schematic diagram of the symmetric micro-supercapacitor with 24-interdigital fingers, and Schematic of the stacked

multilayers of MnOx and gold deposited with electron beam evaporation.

5.3 Results and discussion

5.3.1 Schematics of the micro-supercapacitors

Figure 5. 1a shows the photograph of a real flexible symmetric micro-supercapacitor, which

consists of 24 interdigital fingers as electrodes. As shown in the schematic diagram of Figure 5.

1b, each finger has a length of 4.8 mm, a width of 100 m, and an inter-electrode-distance of 100

m. The SEM images (Figure 5. 2a and b) also confirm the multilayer was uniformly deposited,

with an average particle size of around 15 nm. An AFM image of the actual film roughness is

shown in Figure 5. 2c, which were taken using a Nanoscope III SPM (Digital Instruments). A

mean root square roughness of 1.25 nm of the deposited electrode film was measured,

demonstrating the good smoothness, which explains the mechanical stability and eliminates the

electrochemical dissolution of MnOx into the electrolyte during long-term cycling application.


Figure 5. 2. (a) A low magnification, and (b) high magnification SEM image of MnOx/Au multilayers on PET

substrate, showing the particle size is around 15 nm. (c) AFM image of the MnOx/Au multilayers-electrode showing

film roughness.

5.3.2 Oxidation state of Mn ions

Figure 5. 3. X-ray diffraction pattern of as-prepared MnOx film, which is confirmed to be a mixture of crystalline

MnO2 and Mn3O4.


XRD pattern of the as-prepared MnOx film is depicted in Figure 5. 3, demonstrating that the

MnOx film is a mixture of crystalline MnO2 and Mn3O4.

Figure 5. 4. XPS characterization of MnOx film deposited with electron beam evaporation. The O 1s curve is fitted

into three components: Mn-O-Mn in blue line, Mn-OH in magenta line, and H-O-H in green line.

In order to determine the exact oxidation state of the as-deposited MnOx, the surface of the

hybrid films was investigated by XPS (SPECS PHOIBOS 100), as shown in Figure 5. 4 with a

50 nm-thick MnOx film, in which a sputtering cleaning process was used to remove a surface

layer of about 10 nm to ensure the measuring accuracy. The O 1s (530 eV) and Mn 2p (642 eV)

peaks are attributed to the characteristic peaks of manganese dioxide.[151] The O 1s spectrum

can be fitted into three components, which are related to the Mn-O-Mn bond at 530.2 eV for the

tetravalent oxide, the Mn-OH bond at 531.9 eV for an hydrated trivalent oxide, and a H-O-H

bond at 532.9 eV for residual water in the materials.[151] For the as-prepared MnOx, the

contributions to O 1s peak from Mn-O-Mn component, Mn-OH band and H-O-H component are

79%, 20% and 1%, respectively, indicating the existence of a hydrated form of MnO2. The mean

manganese oxidation state can be calculated according to the following equation:[151]

Mn OMn

OHMnOHMnMnOMn

S

) S(III))S(S(IVStateOx

(5. 1)

where S stands for the signal of the different components of the O 1s spectra. It was calculated to

be about 3.7, which indicates Mn4

ions were dominant in the product. The value of the


manganese oxidation state may facilitate the rapid redox reaction between trivalent and

tetravalent manganese ions due to the mixed valence state in the material, as discussed later.

Furthermore, the Mn 2p core level spectrum has two distinctive peaks at binding energies of

641.4 eV and 653.4 eV corresponding to the spin-orbit doublet of Mn 2p3/2 and Mn 2p1/2,

respectively. The energy separation between O 1s (Mn-O-Mn) and Mn 2p3/2 is 111.2 eV, also

indicating Mn4

ions were dominant in the product.[151] In order to investigate the influence of

gold, a 2 nm MnOx thin film was deposited onto the gold film, and the surface of the MnOx/Au

hybrid thin layer was characterized by XPS. Compared to bare MnOx films, the MnOx/Au hybrid

film exhibits more surface OH oxygen in O 1s (Figure 5. 5), which is probably due to the

influence of gold at the interface.

Figure 5. 5. XPS characterization of bare MnOx film and MnOx/Au film with 2 nm MnOx on top of gold to

investigate the MnOx/Au interface. Compared to bare MnOx film, MnOx/Au film with 2 nm MnOx exhibits more

surface OH oxygen in O 1s (the part circled by blue dashed ellipse), probably due to the gold influence at the

interface.

5.3.3 Electrochemical performance: CV and EIS

Electrochemical properties were characterized by CV and EIS, using Zahner IM 6. Figure 5. 6a

and b show the CV results of MnOx/Au multilayers micro-supercapacitors in the potential

window of 0-0.8 V, with a wide range of scan rates: from 10 mV s1

up to 50 V s1

. Even at the

high scan rate of 50 V s1

, the CV curves retain the symmetric rectangular shape and instant

response upon the reversal of voltage, indicating that the micro-supercapacitor experiences

nearly ideal capacitive behavior. It is therefore concluded that ultrahigh power can be obtained

from this device. The energy storage of MnOx comes from surface adsorption/desorption of


electrolyte cations (here: protons) and also a rapid and highly reversible redox reaction between

MnOx and protons, as follows:[151, 162]

MnOOH eHMnO 2 (5. 2)

In this redox reaction, Mn ions vary between the III and IV oxidation states. According to the

calculated mean manganese oxidation state, the as-prepared MnOx film mainly consists of Mn4

ions, while a small portion of Mn3

ions also exists. The co-existence of both Mn4

and Mn3

ions facilitates the fast, reversible redox reaction between them, thus resulting in nearly ideal

capacitive behaviors.

Figure 5. 6. (a) CV curves of the MnOx/Au multilayers micro-supercapacitor at low scan rates from 10 mV s1

to 1

V s1

, and (b) at high scan rates from 5 V s1

to 50 V s1

. (c) Volumetric capacitance calculated from CV curves at

scan rates (10, 50, 100, 200, 500 and 1000 mV s1

) for MnOx/Au multilayers and bare MnOx micro-supercapacitors.

(d) A linear dependence of the discharge current density on the scan rate up to 7 V s1

.

The volumetric specific capacitance C (F cm3

) of the MnOx/Au multilayers-electrode is

determined using the voltammetric charge (Q) integrated from CV curves. Then the total charge


Q is divided by the volume (V) of the MnOx/Au multilayers in the electrodes and the potential

window E, shown in the equation as follows:[151]

E)Q/(V*C (5. 3)

The calculated volumetric capacitances of MnOx/Au multilayers-electrode vs. scan rates (10,

50, 100, 200, 500 and 1000 mV s1

) are plotted in Figure 5. 6c, where the rate capability can be

seen. The volumetric capacitance of the MnOx/Au multilayers-electrode is calculated to be 78.6

F cm3

at a scan rate of 10 mV s1

, and 32.8 F cm3

at 1 V s1

, resulting in a capacitance retention

ratio of 41.7%. For comparison, the volumetric capacitances vs. scan rates for the bare MnOx

electrode is calculated to be 37.9 F cm3

at a scan rate of 10 mV s1

. With increasing scan rates,

the volumetric capacitance is decreased to be 19.9 F cm3

at a high scan rate of 1 V s1

.

Therefore, the volumetric capacitance of bare MnOx electrode at 1 V s1

retains 52.5% of the

maximum value. Although the retention ratio for MnOx/Au multilayers-electrode decreases, the

volumetric capacitance is still 1.7 times higher than bare MnOx electrodes. The gold films in the

MnOx/Au/MnOx/Au/MnOx sandwich-like multilayers serve as highly conductive bridges to

support the redox active MnOx, resulting in highly improved electrochemical performance. As

can be observed in Figure 5. 6d, a linear dependence of discharge current density on scan rate is

obtained up to 7 V s1

, demonstrating the high power ability for the MnOx/Au multilayers-

electrode. As mentioned above, the energy storage contributions come from both surface

adsorption/desorption of electrolyte cations (H) onto the surface of MnOx/Au multilayers and

the fast reversible redox reactions by means of intercalation/extraction of protons into/out of

MnOx. A control electrolyte of 1 M Li2SO4 solution with a high ionic mobility was also tested

(Figure 5. 7), revealed that supercapacitor with aqueous electrolyte Li2SO4 shows a higher

capacitance than solid electrolyte supercapacitors.

Thin layers of oxides promote maximum pseudocapacitance as the redox reaction initiates at

the surface layer of the oxide in contact with the electrolyte, and is followed by the diffusion of

the electrolyte ions inside the oxide, which is suppressed by the diffusion limit of the ions. This

limitation prevents ions going deep into the oxide in the case of a thick layer, and thus generates

a dead volume of oxide that does not take part in the redox reaction, thereby decreasing the

capacitance per unit volume. In our case, 50 nm-films were used as electrodes, which enable fast


proton diffusion and efficient utilization of active materials, thus leading to the high power

ability.

Figure 5. 7. (a) CV curves of the MnOx/Au multilayers micro-supercapacitor measured in 1 M Li2SO4 at scan rates

from 10 mV s1

to 1 V s1

. (b) Comparison of volumetric capacitance for MnOx/Au multilayers measured in aqueous

(1 M Li2SO4) and gel (H2SO4/PVA) electrolyte at scan rates (10, 50, 100, 200, 500 and 1000 mV s1

).

The enhanced electrochemical performance of the MnOx/Au multilayers micro-supercapacitor

was confirmed by the EIS measurement from 1 MHz to 10 mHz, with a potential amplitude of 5

mV. The intercept of the Nyquist curve with the real axis at high frequencies represents the

equivalent series resistance of the device. As observed in the inset in Figure 5. 8a, due to the

small thickness of the electrodes, the MnOx/Au multilayers micro-supercapacitor shows large

intrinsic resistance of around 240 ohm, which has been reported in previous literature.[101] But

compared to the bare MnOx micro-supercapacitor, the MnOx/Au multilayers micro-

supercapacitor has a lower resistance, which is of great importance since less energy and less

power will be wasted to produce unwanted heat during charge/discharge processes. Figure 5. 8b

compares the Bode plots for bare MnOx and MnOx/Au multilayers micro-supercapacitors. The

capacitor response frequency for both devices at the phase angle of 45º was about 200 Hz.

Therefore, the relaxation time constant 0 was calculated to be 5 ms, showing that a pure

capacitive behavior can be obtained and most of its stored energy is accessible at frequencies

below this frequency. At 10 mHz, the phase angle for both micro-supercapacitors are around

80º, close to 90º for ideal capacitors.


Figure 5. 8. (a) Nyquist plots and (b) Bode plots for both bare MnOx and MnOx/Au multilayers micro-

supercapacitors. Inset of (a) shows the magnified high frequency region. (c) Real part and (d) imaginary part of

complex capacitance for both bare MnOx and MnOx/Au multilayers micro-supercapacitors.

To better understand the difference between two micro-supercapacitors, the evolution of the

complex capacitance was conducted. A supercapacitor can be described by using resistance and

capacitance that are functions of the pulsation ω (here with an amplitude of 5 mV), and

indicated as R (ω) and C (ω). C (ω) is defined as follows:[172]

ω''iCω'CωC (5. 4)

The real and imaginary part of the complex capacitance vs. frequency can be obtained

according to Eq. 5:[172]

2

ωZω

ω''Zω'C

2

ωZω

ω'Zω''C (5. 5)


The low frequency value of the real part capacitance C’ (ω) corresponds to the capacitance of

the cell measured during constant-current discharge. The imaginary part capacitance C’’ (ω)

corresponds to an energy dissipation by an irreversible process. Figure 5. 8c presents the change

in the real part capacitance C’ (ω) vs. frequency for both bare MnOx and MnOx/Au multilayers

micro-supercapacitors. C’ (ω) sharply increases between 1000 Hz and 100 Hz for both micro-

supercapacitors. A difference occurs starting from 100 Hz. The bare MnOx micro-supercapacitor

tends to be less frequency dependent and the capacitance values at a low frequency of 10 mHz

are 41.4 and 58.3 F cm3

for bare MnOx and MnOx/Au multilayers micro-supercapacitors,

respectively, which are consistent with the results obtained from CV measurements (Figure 5.

6c). Figure 5. 8d shows the evolution of the imaginary part of capacitance C’’(ω) vs. frequency

for the two micro-supercapacitors. The relaxation time constant was found to be very similar for

the two micro-supercapacitors, 4.7 ms for the bare MnOx, 5.9 ms for the MnOx/Au multilayers,

consistent with the results obtained from the Bode plots. This value is much lower compared to

literature results: 26 ms for onion-like carbon-based micro-supercapacitors,[145] 700 ms for AC-

based micro-supercapacitor,[145] 500 ms for carbon nanoparticles/MnO2.[165]

5.3.4 Long-term stability, Ragone plot and mechanical flexibility

The long-term cycling stability of MnOx/Au multilayers electrodes was measured with CV at a

scan rate of 1 V s1

over 15 000 cycles (Figure 5. 9a). A decline occurs in the first three thousand

cycles, after that the curve retains a smooth plateau. The loss of capacitance may come from the

electrochemical dissolution of active materials.[94] However, the capacitance retention rate still

reached 74.1% after 15 000 cycles, which is an impressive stability, considering the small

thickness of the electrodes.


Figure 5. 9. (a) The long-term cycling stability of MnOx/Au multilayers micro-supercapacitor, measured at a scan

rate of 1 V s1

. (b) Ragone plot of the micro-supercapacitor, in comparison with other energy storage devices. (c)

Compression setup for strain test. (d) CV curves measured at different bending states, indicated by diameters of

curvature.

The Ragone plot (Figure 5. 9b) compares the volumetric power densities and energy densities

of the MnOx/Au multilayers micro-supercapacitors to the data for other energy storage devices.

The micro-supercapacitor exhibits a maximum volumetric energy density of 1.75 mWh cm3

,

which is two times higher than that reported for commercial activated carbon

supercapacitors,[12] five times higher than that of H-TiO2@MnO2//H-TiO2@C supercapacitors

(PVA/LiCl),[148] two orders of magnitude higher than that of graphene supercapacitors

(PVA/H3PO4),[12] and three orders of magnitude higher than that of aluminum electrolytic

capacitors.[12] The maximum volumetric power density is 3.44 W cm3

, which is five times


higher than that of commercial activated carbon supercapacitors, ten times higher than those of

graphene supercapacitors (PVA/H3PO4)[12] and H-TiO2@MnO2//H-TiO2@C supercapacitors

(PVA/LiCl),[148] and three orders of magnitude higher than that for lithium thin film batteries.

For the mechanical flexibility measurement, the micro-supercapacitor was mounted onto a

compression setup (Figure 5. 9c). The setup have two clamps, one of which can be accurately

moved to apply a compressive strain along the measured supercapacitor device. CV

measurements were conducted at a scan rate of 1 V s1

, with capacitors under various bending

states. As shown in Figure 5. 9d, the curvature radius ranges from 4 mm to 0.5 mm. The CV

curves of the micro-supercapacitor measured without strain and at bending states, all show

similar capacitive behavior, with a capacitance change less than 0.09 %. It is worth noting that

the strain measurement was conducted after the long-term cycling test, demonstrating the high

stability of the micro-supercapacitor. After the strain test (Figure 5. 10), the CV behavior still

remains its rectangular shape.

Figure 5. 10. CV curve of MnOx/Au multilayer micro-supercapacitor measured at a scan rate of 1 V s1

after the

strain test.

5.4 Conclusion

In this work, a new concept is introduced to fabricate on chip, all solid-state and flexible micro-

supercapacitors based on MnOx/Au multilayers, which is compatible with current

microelectronics. The micro-supercapacitor exhibits a maximum energy density of 1.75 mWh

cm3


, which are both much higher than the state

of the art supercapacitors. At a scan rate of 1 V s1

, a volumetric capacitance of 32.8 F cm3

is


obtained for MnOx/Au multilayers electrodes, which is much higher than the bare MnOx

electrode. EIS and the evolution complex capacitance confirm that the electrical conductivity of

MnOx is improved due to incorporation of gold, and a low relaxation time constant around 5 ms

is observed. The MnOx/Au multilayers micro-supercapacitor also shows good long-term cycling

stability, with a capacitance retention rate of 74.1% after a large cycling number of 15 000 times.

Such an architecture enables effective charge transport and electrode integrity, thus endowing the

electrodes with high volumetric capacitance, high energy and power density. The good long-term

cycling stability and high flexibility of the all solid-state micro-supercapacitor show potential

applications in smart environments and miniaturized electronic devices, which require power

resources with small dimensions and high power density.

70 Chapter 6. Fiber-shaped supercapacitors

6. Fiber-shaped supercapacitors based on

Cu wire

The flexibility of energy storage devices normally originates from flexible substrates, including

carbon cloth, PET, PDMS, Teflon plate, paper, yarn, CNT fiber, graphene fiber, rubber fiber,

metal wires, and so on. Compared to old fashioned 2D energy devices, fiber-shaped energy

devices with 1D wire structures have extraordinary advantages such as excellent flexibility and

small scale, which enable the direct integration of them into flexible electronics. Especially metal

wires, which are commonly used as electrical conductors in modern society, can be used as

substrates to build flexible energy storage devices and conduct electricity at the same time.

It is challenging to homogeneously coat active materials onto 1D wires. For example, with

physical vapor deposition, a shadow effect is inevitable, and it is impossible to get a complete

coating on wires during a single deposition. Chemical methods such as hydrothermal method

(carbonization of glucose directly on wires) can homogeneously coat materials, but it is time-

consuming and needs high temperature, while the coating amount is little. A common method is

to dip-coat the wire with appropriate adhesives, but the resulting coating is inhomogeneous. In

this work, a spray coating method followed by a heating process is adopted. Nafion

(Perfluorinated resin solution, 5wt % in lower aliphatic alcohols and water, Aldrich) was used as

adhesive and dispersing agent for MnO2 and/or carbon, which were mixed together in the ratio of

1:1. A gel electrolyte was prepared in the same way as used for the micro-supercapacitor

introduced in Chapter 5, but here the electrolyte was KOH. The schematic configuration of a

supercapacitor is shown in Figure 6. 1.

Chapter 6. Fiber-shaped supercapacitors 71

Figure 6. 1. A schematic of a coaxial supercapacitor based on a Cu wire. Three layers are coated onto the wire, the

inner MnO2/carbon, and the outer carbon work as two electrodes, and the gel electrolyte works in between them.

As discussed in Chapter 2.2.2, the bipolar construction leads to reduced weight, higher

power/energy density with smaller container than conventional monopolar systems. In this work,

the measurement of supercapacitors were realized by twisting two such mono-supercapacitors

together, i.e., in the mode of bipolar construction. Due to the bipolar design, the voltage window

of the supercapacitor can be extended from 0.8 V to 1.5 V, and the capacitive behavior is still

remained. Figure 6. 2a gives the primary results of the fiber-shaped supercapacitor with a length

of 27 cm, as seen in Figure 6. 2b. Further work will be carried out in future.

Figure 6. 2. (a) CV curves of a bipolar supercapacitor at the scan rate of 500 mV s1

within two voltage windows: 0-

0.8 V and 0-1.5 V. (b) A photograph of the wire supercapacitor.

72 Chapter 7. Conclusion and Outlook

7. Conclusion and Outlook

This doctoral thesis is aimed at designing electrochemical energy storage micro-devices: Li-ion

batteries and supercapacitors, which are compatible with current microelectronic techniques.

Interdisciplinary fields are involved in this work including electrochemistry, materials science,

and microelectronic technique, etc.

The first section in this work utilizes the rolled-up nanotechnology approach to fabricate

single silicon rolled-up tubes as anodes for micro-batteries. A lab-on-chip electrochemical device

has been developed, which allows for the study of the lithiation and delithiation process of a

single rolled-up Si tube, the determination of the diffusion coefficient and electrical conductivity,

as well as the characterization of structural changes. A tubular structured anode demonstrates

enhanced diffusion effect, compared to a planar film. Highly wrinkled surfaces of the tube anode

are obtained after charge/discharge cycling. The single rolled-up tube battery described here is

promising for the fundamental research of voltammetry and electrochemical processes and could

also be used as local on-chip energy supply or for driving ultra-compact autonomous

microsystems.

The second part introduces a new concept to fabricate on chip, all solid-state and flexible

micro-supercapacitors based on MnOx/Au multilayers. The micro-supercapacitor exhibits a

maximum energy density of 1.75 mWh cm3


,

which are both much higher than the state of the art supercapacitors. A low relaxation time

constant around 5 ms is observed. The micro-supercapacitor also shows good long-term cycling

stability, with a capacitance retention of 74.1% after a large cycling number of 15 000 times. The

micro-supercapacitor shows potential applications in smart environments and miniaturized

electronic devices, which require power resources with small dimensions and high power

density.

Further work needs to be carried out to optimize the fiber-shaped supercapacitor, which could

be feasibly integrated with other portable electronics or even woven into textiles.

Chapter 7. Conclusion and Outlook 73

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List of Figures and Tables

Figure 1. 1. Sketch of Ragone plot ............................................................................................................... 2

Figure 2. 1. Schematic operating mechanisms of a typical rechargeable LIB ............................................. 7

Figure 2. 2. A schematic drawing of graphene. ........................................................................................... 8

Figure 2. 3. Schematic illustration of Si nanowire anode .......................................................................... 11

Figure 2. 4. Galvanostatic and CV curves recorded on crystalline Si nanowires ..................................... 14

Figure 2. 5. Schematic structures for two groups of cathodes ................................................................... 15

Figure 2. 6. Schematic of measuring a single Si nanowire. ....................................................................... 16

Figure 2. 7. Schematic of contacting a single particle as anode for LIB. .................................................. 17

Figure 2. 8. Schematic of contacting SnO2 nanowires under TEM. ........................................................... 17

Figure 2. 9. Schematic illustration of three types of supercapacitors. ....................................................... 18

Figure 2. 10. Schematic diagrams of double layer capacitor .................................................................... 21

Figure 2. 11. The working principle of pseudocapacitors.. ........................................................................ 21

Figure 2. 12. Equivalent circuit model for a battery-type energy storage system ..................................... 23

Figure 2. 13. Schematic illustrations showing monopolar and bipolar ..................................................... 24

Figure 2. 14. Typical CV curves and galvanostatic charge-discharge curves. .......................................... 26

Figure 2. 15. CV curve of a MnO2 electrode in aqueous electrolyte .......................................................... 27

Figure 2. 16. Various flexible mechanical and electrical sensors ............................................................. 30

Figure 3. 1. Process flow for positioning rolled-up nanomembranes. ....................................................... 34

Figure 3. 2. A scheme showing the method to roll-up a tube ..................................................................... 34

Figure 3. 3. Representative discharge-charge voltage profiles of a 2 m Si film. ..................................... 36

Figure 3. 4. Nyquist impedance plot of an ideal capacitor and an electrochemical capacitor ................. 38

Figure 3. 5. Schematic showing the working principle of XRD ................................................................. 39

Figure 3. 6. Schematic diagrams for the working principle of XPS ........................................................... 40

Figure 3. 7. Schematic energy-level diagram indicating the states involved in Raman signal. ................. 41

Figure 4. 1. A schematic depicting procedures for rolling up a single Si tube .......................................... 45

Figure 4. 2. An encapsulated Li-ion battery with a single silicon tube as anode....................................... 46

Figure 4. 3. Optical micrograph and Raman spectrum of a single rolled-up silicon tube ......................... 47

Figure 4. 4. CV curves of the first three cycles at a scan rate of 100 V s1

.............................................. 49

Figure 4. 5. The current-time response, Cottrell plot and Chemical diffusion coefficient (D).. ................ 50

Figure 4. 6. Typical I-V curves of Si tube and electrical conductivity. ...................................................... 52

Figure 4. 7. SEM images of a tube ............................................................................................................. 53

Figure 5. 1. Photograph and schematic diagram of a flexible micro-supercapacitor. .............................. 58

Figure 5. 2. SEM image and AFM image of MnOx/Au multilayers ............................................................ 59

Figure 5. 3. XRD of as-prepared MnOx film. ............................................................................................. 59

Figure 5. 4. XPS characterization of MnOx ................................................................................................ 60

Figure 5. 5. XPS characterization of bare MnOx film and MnOx/Au film with 2 nm MnOx ....................... 61

Figure 5. 6. CV curves and volumetric capacitance of the micro-supercapacitor. .................................... 62

Figure 5. 7. CV curves of the micro-supercapacitor in 1 M Li2SO4. .......................................................... 64

Figure 5. 8. Impedance plots and complex capacitance for micro-supercapacitors ................................. 65

Figure 5. 9. Long-term stability, Ragone plot and flexibility test ............................................................... 67

Figure 5. 10. CV curve after the strain test. ............................................................................................... 68

Figure 6. 1. A schematic of a coaxial supercapacitor based on a Cu wire ................................................ 71

Figure 6. 2. CV curves and photograph a fiber-shaped supercapacitor. ................................................... 71

Table 2. 1. Summarization of conversion anodes. ...................................................................................... 10

Table 2. 2. Summary of the lithiation/delithiation mechanisms for Si. ....................................................... 13

Table 2. 3 Overall comparison between LIBs and supercapacitors. .......................................................... 31

Versicherung

Hiermit versichere ich, dass ich die vorliegende Arbeit ohne unzulässige Hilfe Dritter und ohne

Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe; die aus fremden Quellen

direkt oder indirekt übernommenen Gedanken sind als solche kenntlich gemacht.

Bei der Auswahl und Auswertung des Materials sowie bei der Herstellung des Manuskripts habe

ich Unterstützungsleistungen von folgenden Personen erhalten:

Prof. Dr. Oliver G. Schmidt

Prof. Dr. Chenglin Yan

Dr. Yao Chen

Xiaolei Sun

Weitere Personen waren an der Abfassung der vorliegenden Arbeit nicht beteiligt. Die Hilfe

eines Promotionsberaters habe ich nicht in Anspruch genommen. Weitere Personen haben von

mir keine geldwerten Leistungen für Arbeiten erhalten, die im Zusammenhang mit dem Inhalt

der vorgelegten Dissertation stehen.

Die Arbeit wurde bisher weder im Inland noch im Ausland in gleicher oder ähnlicher Form einer

anderen Prüfungsbehörde vorgelegt.

........................................... ...............................................

Ort, Datum Unterschrift

Acknowledgements

First and foremost, I would like to acknowledge my family. Especial thanks to my parents for raising me

and being so supportive in every single moment of my life. Thanks to my brother for always being there

for me and encouraging me. No words could properly express what I feel for my family.

I would like to thank the financial support of the International Research Training Group (IRTG) and

PAKT project no. 49004401. I want to sincerely thank all the people who have helped and supported me

during the long way to complete this thesis.

I would like to thank Prof. Dr. Oliver G. Schmidt for giving me the opportunity to pursue my Ph.D. in this

wonderful institute. I also want to thank Prof. Dr. Thomas Geßner for giving me this chance doing my

Ph.D. within the framework of IRTG. And I am very grateful to Prof. Dr. Yongfeng Mei to be a referee of

my doctoral thesis and for introducing me to the scientific research in this institute.

I would like to thank Shilong Li for suggesting I come pursue my Ph.D. in this institute, which was one of

the best decisions I have made in my life. I thank him and Na Zheng for helping me adjust to life here and

their friendship over the years. I would like to greatly thank my group leader Prof. Dr. Chenglin Yan for

his support and advice in scientific research and teaching me to write nice publications. I thank all

colleagues in the energy group for their help and fruitful discussions: Dr. Yao Chen, Dr. Xianghong Liu,

Dr. Lin Zhang, Xiaolei Sun, Junwen Deng, Bo Liu, Xueyi Lu.

And I want to thank Ronny Engelhard, who amazingly can get every machine work. Thanks also to Dr.

Ingolf Mönch, who helped me to overcome the horrible bottleneck at the very beginning of my Ph.D.

Thanks to Gungun Lin, Peixuan Chen and Bezuayehu Teshome for inspiring and saving my research. I

thank all the other colleagues in IIN who are always willing to give me help from all aspects during my

Ph.D. time: Dr. Stefan Baunack, Dr. Daniel Grimm, Dr. Stefan Harazim, Martin Bauer, Sebastian Seifert,

Dr. Luyang Han, Anika Hofmann, Dr. Denys Markarov, Dr. Wang Xi, Britta Koch, Stefan Böttner,

Bianca Höfer, Veronika Magdanz, Abbas Madani, Dr. Fei Ding, Dr. Silvia Giudicatti, Dr. Maria Guix

Noguera, Dr. Yongheng Huo, Dr. Libo Ma, Barbara Eichler, Sandra Nestler, Dr. Dominic Thurmer, Dr.

Lluís Soler Turu, Dr. Honglou Zhen, Dr. Hengxing Ji, Dr. Jianjun Zhang, Jiaxiang Zhang, Dr. Matthew

Jorgensen, Vladimir Bolanos, Dr. Elliot Smith, Dr. Rerngchai Arayanarakool, Daniil Karnaushenko,

Dmitriy Karnaushenko, Dr. Maria Bendova, Dr. Céline Vervacke, Cornelia Krien, Irina Fiering, Michael

Melzer, Robert Streubel, Dr. Samuel Sanchez, Dr. Feng Zhu, Dr. Jianjun Zhang, Eugenio Zallo, Sonja

Maria Marz, Vivienne Meier. Great thanks to Ulrike Steere, Anja Schanze and Grit Rötzer for countless

paper works and caring about me.

I also thank colleagues from IKM who gave me great help with experiments: Dr. Steffen OswaldAndrea

Voß. I also thank the help and friendship from Guozhi Ma, Pan Ma, Lixia Xi, Yandong Jia, Kaikai Song.

I thank the IT people René Pokorny and André Eichler for the support with computers.

I also greatly appreciate the members from IRTG for giving me help and support: Dr. Ramona Ecke, Dr.

Thomas Wächtler, Dr. Knut Schulze, Ms. Katrin Träber. Thanks to my friends from IRTG: Jinji Luo, Dr.

Li Ding, Peng Jiang, Xiao Hu.

I thank all my friends in China and to anyone else who has helped me during the long way. I would never

be able to finish this work without their supports and encouragements.

Publications (Peer-Review)

1. W. Si, C. Yan, Y. Chen, S. Oswald, L. Han, O. G. Schmidt. On chip, all solid-state and flexible

micro-supercapacitors with high performance based on MnOx/Au multilayers. Energy Environ. Sci. 6

(2013) 3218-3223. (back cover)

2. W. Si, X. Sun, X. Liu, L. Xi, Y. Jia, C. Yan, O. G. Schmidt. High areal capacity, micrometer-scale

amorphous Si film anode based on nanostructured Cu foil for Li-ion batteries. J. Power Sources 267

(2014) 629-634.

3. W. Si, I. J. Mönch, C. Yan, J. Deng, S. Li, G. Lin, L. Han, Y. Mei, O. G. Schmidt. A single rolled-up

Si tube battery for study of electrochemical kinetics, electrical conductivity and structural integrity.

Adv. Mater. 26 (2014) 7973-7978.

4. X. Sun, W. Si, X. Liu, J. Deng, L. Xi, L. Liu, C. Yan, O. G. Schmidt. Multifunctional Ni/NiO

hybridized nanomembranes as anode materials for high-rate Li-ion batteries. Nano Energy 9 (2014)

168-175.

5. X. Liu, W. Si, X. Sun, J. Deng, J. Zhang, S. Baunack, S. Oswald, L. Liu, C. Yan, O. G. Schmidt.

Free-standing Fe2O3 nanomembranes as anodes for Li-ion batteries with long cycling life and high

rate capability. Sci. Rep. 4 (2014) 7452.

6. E. Song, W. Si, R. Cao, P. Feng, I. Mönch, G. Huang, Z. Di, O. G. Schmidt, Y. Mei. Schottky contact

on ultra-thin silicon nanomembranes under light illumination. Nanotechnology 25 (2014) 485201.

7. X. Liu, J. Zhang, W. Si, S. Oswald, C. Yan, O. G. Schmidt. High-rate amorphous SnO2

nanomembrane anodes for Li-ion batteries with a long cycling life. Nanoscale 7 (2015) 282-288.

8. C. Yan, W. Xi, W. Si, J. Deng, O. G. Schmidt. Highly conductive and strain-released hybrid

multilayer Ge/Ti nanomembranes with enhanced lithium-ion-storage capability. Adv. Mater. 25

(2013) 539-544.

9. X. Liu, J. Zhang, W. Si, L. Xi, S. Oswald, C. Yan, O. G. Schmidt. High-rate amorphous SnO2

nanomembrane anodes for Li-ion batteries with a long cycling life. Nanoscale 7 (2015) 282.

10. X. Sun, C. Yan, Y. Chen, W. Si, J. Deng, S. Oswald, L. Liu, O. G. Schmidt. Three-dimensionally

“curved”NiO nanomembranes as ultrahigh rate capability anodes for Li-ion batteries with long cycle

lifetimes. Adv. Energy Mater. 4 (2014) 1300912.

11. L. Zhang, J. Deng, L. Liu, W. Si, S. Oswald, L. Xi, M. Kundu, G. Ma, T. Gemming, S. Baunack, F.

Ding, C. Yan, O. G. Schmidt. Hierarchically designed SiOx/SiOy bilayer nanomembranes as stable

anodes for lithium ion batteries. Adv. Mater. 26 (2014) 4527-4532.

12. G. Lin, D. Makarov, M. Melzer, W. Si, C. Yan, O. G.Schmidt. A highly flexible and compact

magnetoresistive analytic device. Lab chip 14 (2014) 4050-4057.

13. J. Deng, H. Ji, C. Yan, J. Zhang, W. Si, S. Baunack, S. Oswald, Y. Mei, O. G. Schmidt. Naturally

rolled-up C/Si/C trilayer nanomembranes as stable anodes for lithium-ion batteries with remarkable

cycling performance. Angew. Chem. Int. Ed. 52 (2013) 2326-2330.

Conferences and scientific presentations

1. W. Si, C. Yan, O. G. Schmidt

Single rolled-up microtube lithium ion batteries and flexible solid-state micro-supercapacitors

International Research Training Group Summer School Shanghai, Shanghai, 24-28 June 2013, oral


Flexible, on-chip, all solid state micro-supercapacitors based on MnOx/Au multilayers

IFW winter school, Oberwiesenthal, January 20-23, 2013, poster

3. 5th

International Workshop on Impedance Spectroscopy, Chemnitz, 26-28 September 2012


Single rolled-up microtube electrochemical devices

International Research Training Group Summer School Zeuthen, Germany, 22-27 July 2012, oral


Electrochemical energy storage micro-devices based on MnOx/Au multilayers

Nano- and Bio-techniques for the packaging of electronic systems, Chemnitz, 2-5 May 2012, poster

6. W. Si, Y. Mei, O. G. Schmidt

Impedance spectroscopy investigation of semiconductor devices based on SOI

International Research Training Group Summer School Shanghai, 5-12 April 2011, Shanghai, oral

Cover pages in journals

1. Energy Environ. Sci. 6, 2013, 3218-3223. (back cover)

2. Adv. Mater., 2014, received invitation, draft ready to submit for consideration. (29.09.2014)

Grants and contributions to research groups

1. Contribution to the DFG research group IRTG Project GDK 1215/2 “Rolled-up nanotech for on-

chip energy storage”.

2. Participation on projects PAKT project “Electrochemical energy storage in autonomous systems,

no. 49004401”.

3. 2014 Chinese Government Award for Outstanding Self-financed Students Abroad, submitted for

consideration.

Curriculum Vitae

Name: Wenping Si Nürnberger Strasse 28g

Date of Birth: 16th August 1986 01187, Dresden

Nationality: Chinese Phone: +49-351-4659-869

Gender: female Email: [email protected]

Education 10/2010 – today Ph.D. student in Leibniz Institute for Solid State and Materials Research Dresden

(IFW), and Chemnitz University of Technology, Germany; 03/2011-06/2011:

guest Ph.D. student in Fudan University, China

09/2008 – 06/2010 Master of Materials Science in Jilin University, China

09/2004 – 06/2008 Bachelor of Materials Science in Jilin University, China

Research Experience 10/2010 – today Ph.D. thesis on “Electrochemical energy storage: lithium-ion batteries and

supercapacitors” in Leibniz Institute for Solid State and Materials Research

Dresden (IFW), and Chemnitz University of Technology, Germany

09/2008 – 06/2010 Graduate Research Program “First-principles study on the elastic property and

electronic structure of Ti5Si3 doped with third element” in Jilin University, China

09/2007 – 02/2008 Undergraduate research “Preparation and modification of Kaolin” in Jilin

University, China

Language Skills Mandarin Chinese native

English fluent

German middle

Korean basic knowledge

Honors 06/2009 The 2

nd class Outstanding Scholarship (Top 13%), Jilin University.

06/2008 The 2nd

class Outstanding Scholarship (Top 13%) & “Excellent Student” of Dept.

of Materials Science and Engineering, Jilin University.

06/2007 The 2nd


of Materials Science and Engineering, Jilin University.

06/2006 The 1st class Outstanding Scholarship (Top 5%) & “Excellent Student” of Jilin

University.

06/2005 The 2nd


of Material Science and Engineering, Jilin University.

mailto:[email protected]

Documents

Designing Electrochemical Energy Storage Microdevices: Li